diff --git a/specs/data/combineTilesets/externalInSameDirectory/README.md b/specs/data/combineTilesets/externalInSameDirectory/README.md
new file mode 100644
index 0000000..865db03
--- /dev/null
+++ b/specs/data/combineTilesets/externalInSameDirectory/README.md
@@ -0,0 +1,18 @@
+A regression test for https://github.com/CesiumGS/3d-tiles-tools/issues/138 
+
+```
+tileset.json refers to
+  sub0/externalA.json
+  
+sub0/externalA.json refers to
+  tileA.b3dm
+  externalB.json
+  
+sub0/externalB.json refers to
+  tileB.b3dm
+```  
+
+The content URI for `tileB.b3dm` of the combined result should be `sub0/tileB.b3dm`
+
+  
+  
\ No newline at end of file
diff --git a/specs/data/combineTilesets/externalInSameDirectory/sub0/externalA.json b/specs/data/combineTilesets/externalInSameDirectory/sub0/externalA.json
new file mode 100644
index 0000000..dedb655
--- /dev/null
+++ b/specs/data/combineTilesets/externalInSameDirectory/sub0/externalA.json
@@ -0,0 +1,32 @@
+{
+  "asset": {
+    "version": "1.1"
+  },
+  "geometricError": 4.0,
+  "root": {
+    "boundingVolume": {
+      "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+    },
+    "geometricError": 2.0,
+    "children": [
+      {
+        "boundingVolume": {
+          "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+        },
+        "geometricError": 1.0,
+        "content": {
+          "uri": "tileA.b3dm"
+        }
+      },
+      {
+        "boundingVolume": {
+          "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+        },
+        "geometricError": 1.0,
+        "content": {
+          "uri": "externalB.json"
+        }
+      }
+    ]
+  }
+}
diff --git a/specs/data/combineTilesets/externalInSameDirectory/sub0/externalB.json b/specs/data/combineTilesets/externalInSameDirectory/sub0/externalB.json
new file mode 100644
index 0000000..ce6f5bb
--- /dev/null
+++ b/specs/data/combineTilesets/externalInSameDirectory/sub0/externalB.json
@@ -0,0 +1,23 @@
+{
+  "asset": {
+    "version": "1.1"
+  },
+  "geometricError": 4.0,
+  "root": {
+    "boundingVolume": {
+      "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+    },
+    "geometricError": 2.0,
+    "children": [
+      {
+        "boundingVolume": {
+          "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+        },
+        "geometricError": 1.0,
+        "content": {
+          "uri": "tileB.b3dm"
+        }
+      }
+    ]
+  }
+}
diff --git a/specs/data/combineTilesets/externalInSameDirectory/sub0/tileA.b3dm b/specs/data/combineTilesets/externalInSameDirectory/sub0/tileA.b3dm
new file mode 100644
index 0000000..e69de29
diff --git a/specs/data/combineTilesets/externalInSameDirectory/sub0/tileB.b3dm b/specs/data/combineTilesets/externalInSameDirectory/sub0/tileB.b3dm
new file mode 100644
index 0000000..e69de29
diff --git a/specs/data/combineTilesets/externalInSameDirectory/tileset.json b/specs/data/combineTilesets/externalInSameDirectory/tileset.json
new file mode 100644
index 0000000..5324623
--- /dev/null
+++ b/specs/data/combineTilesets/externalInSameDirectory/tileset.json
@@ -0,0 +1,23 @@
+{
+  "asset": {
+    "version": "1.1"
+  },
+  "geometricError": 4.0,
+  "root": {
+    "boundingVolume": {
+      "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+    },
+    "geometricError": 2.0,
+    "children": [
+      {
+        "boundingVolume": {
+          "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+        },
+        "geometricError": 1.0,
+        "content": {
+          "uri": "sub0/externalA.json"
+        }
+      }
+    ]
+  }
+}
diff --git a/specs/data/combineTilesets/externalMultipleContents/README.md b/specs/data/combineTilesets/externalMultipleContents/README.md
new file mode 100644
index 0000000..715b22c
--- /dev/null
+++ b/specs/data/combineTilesets/externalMultipleContents/README.md
@@ -0,0 +1,28 @@
+A test for the handling of multiple contents in the `combine` command
+(Brought up in https://github.com/CesiumGS/3d-tiles-tools/issues/138 )
+
+The cases that are differentiated here:
+
+- A tile refers to a SINGLE content, which is an external tileset
+  - In this case, the properties of the tile will be "replaced" with the properties of the external root
+- A tile refers to an multiple contents, including external tilesets
+  - In this case, the external roots will be added as children to the tile
+
+---
+
+- The `tileset.json` root 
+  - No children
+  - Multiple contents: `tile.b3dm`, `/subA/externalA.json`, `/subB/externalB.json`
+- The result root should have one content and two children
+
+- The `/subA/externalA.json` root:
+  - Multiple contents: `tileA0.b3dm`, `tileA1.b3dm`
+  - No content
+- This should be the first child of the result root
+
+- The `/subB/externalB.json` root: 
+  - A single content: `externalB0.json`
+  - No children
+- The `externalB0.json` contains two contents, `tileB0x.b3dm` and `tileB0y.b3dm`. 
+- These should be put into root of the `/subB/externalB.json`
+- This will be the second child of the result root
diff --git a/specs/data/combineTilesets/externalMultipleContents/subA/externalA.json b/specs/data/combineTilesets/externalMultipleContents/subA/externalA.json
new file mode 100644
index 0000000..e1ce060
--- /dev/null
+++ b/specs/data/combineTilesets/externalMultipleContents/subA/externalA.json
@@ -0,0 +1,20 @@
+{
+  "asset": {
+    "version": "1.1"
+  },
+  "geometricError": 4.0,
+  "root": {
+    "boundingVolume": {
+      "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+    },
+    "geometricError": 2.0,
+    "contents": [
+      {
+        "uri": "tileA0.b3dm"
+      },
+      {
+        "uri": "tileA1.b3dm"
+      }
+    ]
+  }
+}
diff --git a/specs/data/combineTilesets/externalMultipleContents/subA/tileA0.b3dm b/specs/data/combineTilesets/externalMultipleContents/subA/tileA0.b3dm
new file mode 100644
index 0000000..e69de29
diff --git a/specs/data/combineTilesets/externalMultipleContents/subA/tileA1.b3dm b/specs/data/combineTilesets/externalMultipleContents/subA/tileA1.b3dm
new file mode 100644
index 0000000..e69de29
diff --git a/specs/data/combineTilesets/externalMultipleContents/subB/externalB.json b/specs/data/combineTilesets/externalMultipleContents/subB/externalB.json
new file mode 100644
index 0000000..18498c9
--- /dev/null
+++ b/specs/data/combineTilesets/externalMultipleContents/subB/externalB.json
@@ -0,0 +1,15 @@
+{
+  "asset": {
+    "version": "1.1"
+  },
+  "geometricError": 4.0,
+  "root": {
+    "boundingVolume": {
+      "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+    },
+    "geometricError": 2.0,
+    "content": {
+      "uri": "externalB0.json"
+    }
+  }
+}
diff --git a/specs/data/combineTilesets/externalMultipleContents/subB/externalB0.json b/specs/data/combineTilesets/externalMultipleContents/subB/externalB0.json
new file mode 100644
index 0000000..886e0a7
--- /dev/null
+++ b/specs/data/combineTilesets/externalMultipleContents/subB/externalB0.json
@@ -0,0 +1,20 @@
+{
+  "asset": {
+    "version": "1.1"
+  },
+  "geometricError": 4.0,
+  "root": {
+    "boundingVolume": {
+      "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+    },
+    "geometricError": 2.0,
+    "contents": [
+      {
+        "uri": "tileB0x.b3dm"
+      },
+      {
+        "uri": "tileB0y.b3dm"
+      }
+    ]
+  }
+}
diff --git a/specs/data/combineTilesets/externalMultipleContents/subB/tileB0x.b3dm b/specs/data/combineTilesets/externalMultipleContents/subB/tileB0x.b3dm
new file mode 100644
index 0000000..e69de29
diff --git a/specs/data/combineTilesets/externalMultipleContents/subB/tileB0y.b3dm b/specs/data/combineTilesets/externalMultipleContents/subB/tileB0y.b3dm
new file mode 100644
index 0000000..e69de29
diff --git a/specs/data/combineTilesets/externalMultipleContents/tile.b3dm b/specs/data/combineTilesets/externalMultipleContents/tile.b3dm
new file mode 100644
index 0000000..e69de29
diff --git a/specs/data/combineTilesets/externalMultipleContents/tileset.json b/specs/data/combineTilesets/externalMultipleContents/tileset.json
new file mode 100644
index 0000000..0470808
--- /dev/null
+++ b/specs/data/combineTilesets/externalMultipleContents/tileset.json
@@ -0,0 +1,23 @@
+{
+  "asset": {
+    "version": "1.1"
+  },
+  "geometricError": 4.0,
+  "root": {
+    "boundingVolume": {
+      "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+    },
+    "geometricError": 2.0,
+    "contents": [
+      {
+        "uri": "tile.b3dm"
+      },
+      {
+        "uri": "subA/externalA.json"
+      },
+      {
+        "uri": "subB/externalB.json"
+      }
+    ]
+  }
+}
diff --git a/specs/data/combineTilesets/externalWithUrl/README.md b/specs/data/combineTilesets/externalWithUrl/README.md
new file mode 100644
index 0000000..e896c30
--- /dev/null
+++ b/specs/data/combineTilesets/externalWithUrl/README.md
@@ -0,0 +1,5 @@
+A test for the `combine` functionality based on a tileset that uses
+the (legacy) `url` property for its contents.
+
+See https://github.com/CesiumGS/3d-tiles-tools/issues/43
+
diff --git a/specs/data/combineTilesets/externalWithUrl/external.json b/specs/data/combineTilesets/externalWithUrl/external.json
new file mode 100644
index 0000000..fce558a
--- /dev/null
+++ b/specs/data/combineTilesets/externalWithUrl/external.json
@@ -0,0 +1,23 @@
+{
+  "asset": {
+    "version": "1.0"
+  },
+  "geometricError": 4.0,
+  "root": {
+    "boundingVolume": {
+      "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+    },
+    "geometricError": 2.0,
+    "children": [
+      {
+        "boundingVolume": {
+          "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+        },
+        "geometricError": 1.0,
+        "content": {
+          "url": "tile.b3dm"
+        }
+      }
+    ]
+  }
+}
diff --git a/specs/data/combineTilesets/externalWithUrl/tile.b3dm b/specs/data/combineTilesets/externalWithUrl/tile.b3dm
new file mode 100644
index 0000000..e69de29
diff --git a/specs/data/combineTilesets/externalWithUrl/tileset.json b/specs/data/combineTilesets/externalWithUrl/tileset.json
new file mode 100644
index 0000000..f701527
--- /dev/null
+++ b/specs/data/combineTilesets/externalWithUrl/tileset.json
@@ -0,0 +1,23 @@
+{
+  "asset": {
+    "version": "1.0"
+  },
+  "geometricError": 4.0,
+  "root": {
+    "boundingVolume": {
+      "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+    },
+    "geometricError": 2.0,
+    "children": [
+      {
+        "boundingVolume": {
+          "box": [0.5, 0.5, 0.5, 0.5, 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.0, 0.5]
+        },
+        "geometricError": 1.0,
+        "content": {
+          "url": "external.json"
+        }
+      }
+    ]
+  }
+}
diff --git a/specs/tools/tilesetProcessing/TilesetCombinerSpec.ts b/specs/tools/tilesetProcessing/TilesetCombinerSpec.ts
index f4d4860..a6f0e71 100644
--- a/specs/tools/tilesetProcessing/TilesetCombinerSpec.ts
+++ b/specs/tools/tilesetProcessing/TilesetCombinerSpec.ts
@@ -7,17 +7,9 @@ import { TilesetOperations } from "../../../src/tools";
 import { SpecHelpers } from "../../SpecHelpers";
 
 const SPECS_DATA_BASE_DIRECTORY = SpecHelpers.getSpecsDataBaseDirectory();
-
-const nestedExteralInput =
-  SPECS_DATA_BASE_DIRECTORY + "/combineTilesets/nestedExternal";
-const nestedExteralOutput =
-  SPECS_DATA_BASE_DIRECTORY + "/output/combineTilesets/nestedExternal";
-
-const externalsWithTransformInput =
-  SPECS_DATA_BASE_DIRECTORY + "/combineTilesets/externalsWithTransform";
-const externalsWithTransformOutput =
-  SPECS_DATA_BASE_DIRECTORY + "/output/combineTilesets/externalsWithTransform";
-
+const INPUT_BASE_DIRECTORY = SPECS_DATA_BASE_DIRECTORY + "/combineTilesets/";
+const OUTPUT_BASE_DIRECTORY =
+  SPECS_DATA_BASE_DIRECTORY + "/output/combineTilesets/";
 const overwrite = true;
 
 describe("TilesetCombiner", function () {
@@ -27,17 +19,16 @@ describe("TilesetCombiner", function () {
     );
   });
 
-  it("combines external tilesets into a single tileset", async function () {
-    await TilesetOperations.combine(
-      nestedExteralInput,
-      nestedExteralOutput,
-      overwrite
-    );
+  it("combines external tilesets into a single tileset in nestedExternal", async function () {
+    const testDirectoryName = "nestedExternal";
+    const input = INPUT_BASE_DIRECTORY + testDirectoryName;
+    const output = OUTPUT_BASE_DIRECTORY + testDirectoryName;
+
+    await TilesetOperations.combine(input, output, overwrite);
 
     // Ensure that the output directory contains the expected files:
     // All files of the input, except for the external tileset JSON files
-    const actualRelativeFiles =
-      SpecHelpers.collectRelativeFileNames(nestedExteralOutput);
+    const actualRelativeFiles = SpecHelpers.collectRelativeFileNames(output);
     actualRelativeFiles.sort();
     const expectedRelativeFiles = [
       "README.md",
@@ -54,7 +45,7 @@ describe("TilesetCombiner", function () {
     // Ensure that the single 'tileset.json' contains the
     // proper content URIs for the combined output
     const tilesetJsonBuffer = fs.readFileSync(
-      Paths.join(nestedExteralOutput, "tileset.json")
+      Paths.join(output, "tileset.json")
     );
     const tileset = JSON.parse(tilesetJsonBuffer.toString());
     const actualContentUris = await SpecHelpers.collectExplicitContentUris(
@@ -73,17 +64,17 @@ describe("TilesetCombiner", function () {
     expect(actualContentUris).toEqual(expectedContentUris);
   });
 
-  it("retains the transforms of root nodes of external tilesets", async function () {
-    await TilesetOperations.combine(
-      externalsWithTransformInput,
-      externalsWithTransformOutput,
-      overwrite
-    );
+  it("retains the transforms of root nodes of external tilesets in externalsWithTransform", async function () {
+    const testDirectoryName = "externalsWithTransform";
+    const input = INPUT_BASE_DIRECTORY + testDirectoryName;
+    const output = OUTPUT_BASE_DIRECTORY + testDirectoryName;
+
+    await TilesetOperations.combine(input, output, overwrite);
 
     // Ensure that the resulting tileset JSON contains the
     // proper content transforms and bounding volumes
     const tilesetJsonBuffer = fs.readFileSync(
-      Paths.join(externalsWithTransformOutput, "tileset.json")
+      Paths.join(output, "tileset.json")
     );
     const tileset = JSON.parse(tilesetJsonBuffer.toString());
 
@@ -122,4 +113,110 @@ describe("TilesetCombiner", function () {
     const childTransformB = tileset.root.children[2].transform;
     expect(childTransformB).toEqual(expectedTransformB);
   });
+
+  it("generates proper content URIs for externalInSameDirectory", async function () {
+    const testDirectoryName = "externalInSameDirectory";
+    const input = INPUT_BASE_DIRECTORY + testDirectoryName;
+    const output = OUTPUT_BASE_DIRECTORY + testDirectoryName;
+
+    await TilesetOperations.combine(input, output, overwrite);
+
+    // Ensure that the output directory contains the expected files:
+    // All files of the input, except for the external tileset JSON files
+    const actualRelativeFiles = SpecHelpers.collectRelativeFileNames(output);
+    actualRelativeFiles.sort();
+    const expectedRelativeFiles = [
+      "README.md",
+      "sub0/tileA.b3dm",
+      "sub0/tileB.b3dm",
+      "tileset.json",
+    ];
+    expect(actualRelativeFiles).toEqual(expectedRelativeFiles);
+
+    // Ensure that the single 'tileset.json' contains the
+    // proper content URIs for the combined output
+    const tilesetJsonBuffer = fs.readFileSync(
+      Paths.join(output, "tileset.json")
+    );
+    const tileset = JSON.parse(tilesetJsonBuffer.toString());
+    const actualContentUris = await SpecHelpers.collectExplicitContentUris(
+      tileset.root
+    );
+    actualContentUris.sort();
+
+    const expectedContentUris = ["sub0/tileA.b3dm", "sub0/tileB.b3dm"];
+    expect(actualContentUris).toEqual(expectedContentUris);
+  });
+
+  it("handles multiple contents with external tilesets in externalMultipleContents", async function () {
+    const testDirectoryName = "externalMultipleContents";
+    const input = INPUT_BASE_DIRECTORY + testDirectoryName;
+    const output = OUTPUT_BASE_DIRECTORY + testDirectoryName;
+
+    await TilesetOperations.combine(input, output, overwrite);
+
+    // Ensure that the output directory contains the expected files:
+    // All files of the input, except for the external tileset JSON files
+    const actualRelativeFiles = SpecHelpers.collectRelativeFileNames(output);
+    actualRelativeFiles.sort();
+    const expectedRelativeFiles = [
+      "README.md",
+      "subA/tileA0.b3dm",
+      "subA/tileA1.b3dm",
+      "subB/tileB0x.b3dm",
+      "subB/tileB0y.b3dm",
+      "tile.b3dm",
+      "tileset.json",
+    ];
+    expect(actualRelativeFiles).toEqual(expectedRelativeFiles);
+
+    // Ensure that the single 'tileset.json' contains the
+    // proper content URIs for the combined output
+    const tilesetJsonBuffer = fs.readFileSync(
+      Paths.join(output, "tileset.json")
+    );
+    const tileset = JSON.parse(tilesetJsonBuffer.toString());
+    const actualContentUris = await SpecHelpers.collectExplicitContentUris(
+      tileset.root
+    );
+    actualContentUris.sort();
+
+    const expectedContentUris = [
+      "subA/tileA0.b3dm",
+      "subA/tileA1.b3dm",
+      "subB/tileB0x.b3dm",
+      "subB/tileB0y.b3dm",
+      "tile.b3dm",
+    ];
+    expect(actualContentUris).toEqual(expectedContentUris);
+  });
+
+  it("handles legacy 'url' properties for content", async function () {
+    const testDirectoryName = "externalWithUrl";
+    const input = INPUT_BASE_DIRECTORY + testDirectoryName;
+    const output = OUTPUT_BASE_DIRECTORY + testDirectoryName;
+
+    await TilesetOperations.combine(input, output, overwrite);
+
+    // Ensure that the output directory contains the expected files:
+    // All files of the input, except for the external tileset JSON files
+    const actualRelativeFiles = SpecHelpers.collectRelativeFileNames(output);
+    actualRelativeFiles.sort();
+    const expectedRelativeFiles = ["README.md", "tile.b3dm", "tileset.json"];
+    expect(actualRelativeFiles).toEqual(expectedRelativeFiles);
+
+    // Ensure that the single 'tileset.json' contains the
+    // proper content URIs for the combined output
+    const tilesetJsonBuffer = fs.readFileSync(
+      Paths.join(output, "tileset.json")
+    );
+    const tileset = JSON.parse(tilesetJsonBuffer.toString());
+    const actualContentUris = await SpecHelpers.collectExplicitContentUris(
+      tileset.root
+    );
+    actualContentUris.sort();
+
+    const expectedContentUris = ["tile.b3dm"];
+    expect(actualContentUris).toEqual(expectedContentUris);
+  });
 });
diff --git a/src/cli/main.ts b/src/cli/main.ts
index a5137ac..bd2dc99 100644
--- a/src/cli/main.ts
+++ b/src/cli/main.ts
@@ -447,6 +447,8 @@ async function run() {
   await runCommand(command, parsedToolArgs, optionArgs);
   const afterMs = performance.now();
   logger.info(`Total: ${(afterMs - beforeMs).toFixed(3)} ms`);
+  // This is not part of the Logger API!:
+  logger.flush();
 }
 
 async function runCommand(command: string, toolArgs: any, optionArgs: any) {
diff --git a/src/tools/tilesetProcessing/TilesetCombiner.ts b/src/tools/tilesetProcessing/TilesetCombiner.ts
index 541a815..3a17426 100644
--- a/src/tools/tilesetProcessing/TilesetCombiner.ts
+++ b/src/tools/tilesetProcessing/TilesetCombiner.ts
@@ -17,6 +17,9 @@ import { TilesetTargets } from "../../tilesets";
 import { Tiles } from "../../tilesets";
 import { Tilesets } from "../../tilesets";
 
+import { Loggers } from "../../base";
+const logger = Loggers.get("tilesetProcessing");
+
 /**
  * A class for combining external tileset of a given tileset, to
  * create a new, combined tileset.
@@ -78,6 +81,10 @@ export class TilesetCombiner {
     tilesetTargetName: string,
     overwrite: boolean
   ): Promise<void> {
+    logger.debug(`Running combine`);
+    logger.debug(`  tilesetSourceName: ${tilesetSourceName}`);
+    logger.debug(`  tilesetTargetName: ${tilesetTargetName}`);
+
     const tilesetSource = TilesetSources.createAndOpen(tilesetSourceName);
     const tilesetTarget = TilesetTargets.createAndBegin(
       tilesetTargetName,
@@ -105,6 +112,8 @@ export class TilesetCombiner {
 
     this.tilesetSource = undefined;
     this.tilesetTarget = undefined;
+
+    logger.debug(`Running combine DONE`);
   }
 
   /**
@@ -140,7 +149,7 @@ export class TilesetCombiner {
     const tileset = JSON.parse(tilesetJsonBuffer.toString()) as Tileset;
 
     this.externalTilesetFileNames.length = 0;
-    await this.combineTilesetsInternal(".", tileset, undefined);
+    await this.combineTilesetsInternal(".", tileset);
 
     this.copyResources(tilesetTargetJsonFileName);
 
@@ -164,26 +173,36 @@ export class TilesetCombiner {
    *
    * @param currentDirectory - The current directory
    * @param tileset - The current tileset
-   * @param parentTile - The optional parent tile
    */
   private async combineTilesetsInternal(
     currentDirectory: string,
-    tileset: Tileset,
-    parentTile: Tile | undefined
+    tileset: Tileset
   ): Promise<void> {
+    // Traverse the tileset, depth-first, and call `combineTileInternal`
+    // on each tile. If the tile contains a content with an external
+    // tileset, then this external tileset will be "inlined". Note
+    // that depending on the structure of the tileset, this may cause
+    // new children to be added to the tile. This has to be taken
+    // into account here (and this is the reason why this cannot
+    // be implemented with `Tiles.traverseExplicit`).
     const root = tileset.root;
-    if (parentTile) {
-      parentTile.content = root.content;
-      parentTile.contents = root.contents;
-      parentTile.children = root.children;
-      parentTile.boundingVolume = root.boundingVolume;
-      parentTile.transform = root.transform;
-    }
-    await Tiles.traverseExplicit(root, async (tilePath: Tile[]) => {
-      const tile = tilePath[tilePath.length - 1];
+    const stack: Tile[] = [];
+    stack.push(root);
+    while (stack.length > 0) {
+      const tile = stack[stack.length - 1];
+      stack.pop();
+
+      // Create a copy of the children array, to handle the
+      // case that new children are added to the tile while
+      // "inlining" the external tilesets.
+      const children = tile.children?.slice();
       await this.combineTileInternal(currentDirectory, tile);
-      return true;
-    });
+      if (children) {
+        for (const child of children) {
+          stack.push(child);
+        }
+      }
+    }
   }
 
   /**
@@ -201,44 +220,71 @@ export class TilesetCombiner {
     tile: Tile
   ): Promise<void> {
     if (tile.content) {
-      await this.combineContentInternal(currentDirectory, tile, tile.content);
+      await this.combineSingleContentInternal(currentDirectory, tile);
     } else if (tile.contents) {
-      for (const content of tile.contents) {
-        await this.combineContentInternal(currentDirectory, tile, content);
-      }
+      await this.combineMultipleContentsInternal(currentDirectory, tile);
     }
   }
 
   /**
-   * This is called for each content of each tile of the source tileset and
-   * all of its external tilesets.
+   * Processes an external tileset that was found during the traversal.
+   * This will be called recursively on all external tilesets. It will
+   * return the external tileset, AFTER it has itself been combined
+   * by passing it to `combineTilesetsInternal`
    *
-   * If the given content points to an external tileset, it is inlined
-   * by calling `combineTilesetsInternal` with the directory and JSON of the
-   * external tileset.
+   * @param externalFileName - The full file name of the external tileset
+   * @param externalFileBuffer - The buffer containing the JSON of the
+   * external tileset
+   * @returns The processed external tileset
+   */
+  private async processExternalTileset(
+    externalFileName: string,
+    externalFileBuffer: Buffer
+  ) {
+    this.externalTilesetFileNames.push(externalFileName);
+    const externalTilesetDirectory = path.dirname(externalFileName);
+    const externalTileset = JSON.parse(
+      externalFileBuffer.toString()
+    ) as Tileset;
+    await this.combineTilesetsInternal(
+      externalTilesetDirectory,
+      externalTileset
+    );
+    return externalTileset;
+  }
+
+  /**
+   * This is called for each tile of the source tileset that has a single
+   * content.
+   *
+   * If the content points to an external tileset, then
+   * - The external tileset is 'combined' by calling `processExternalTileset`
+   * - The properties of the given tile are replaced with the properties of
+   *   the root of the (combined) external tileset.
    *
    * Otherwise, the URL of the content is updated to be relative to
    * the root of the resulting combined tileset.
    *
    * @param currentDirectory - The current directory (see `combineTilesetsInternal`)
    * @param tile - The current tile
-   * @param content - The current tile content
    */
-  private async combineContentInternal(
+  private async combineSingleContentInternal(
     currentDirectory: string,
-    tile: Tile,
-    content: Content
+    tile: Tile
   ): Promise<void> {
     if (!this.tilesetSource || !this.tilesetTarget) {
       throw new DeveloperError("The source and target must be defined");
     }
-
-    const contentUri = content.uri;
-    if (!contentUri) {
-      // This is the case for legacy data (including some of the
-      // original spec data), so handle this case explicitly here.
-      throw new TilesetError("Content does not have a URI");
+    const content = tile.content;
+    if (!content) {
+      throw new DeveloperError(
+        "This should only be called for tiles with a single content"
+      );
     }
+
+    // Obtain the data of the content, and determine whether it is
+    // an external tileset
+    const contentUri = TilesetCombiner.obtainContentUri(content);
     const externalFileName = Paths.join(currentDirectory, contentUri);
     const externalFileBuffer = this.tilesetSource.getValue(externalFileName);
     if (!externalFileBuffer) {
@@ -252,29 +298,149 @@ export class TilesetCombiner {
       // will end up in
       const newUri = Paths.relativize(".", externalFileName);
       content.uri = newUri;
-    } else {
-      // When the data is an external tileset, recursively combine
-      // ("inline") that tileset, and insert its content, contents
-      // and children into the current tile
-      this.externalTilesetFileNames.push(externalFileName);
-      const externalTilesetDirectory = path.dirname(externalFileName);
-      const externalTilesetBuffer =
-        this.tilesetSource.getValue(externalFileName);
-      if (!externalTilesetBuffer) {
-        throw new TilesetError(
-          `Could not obtain data for external ` +
-            `tileset file ${externalFileName}`
-        );
+      return;
+    }
+
+    // The data is an external tileset. Process (combine) this
+    // tileset, recursively
+    const externalTileset = await this.processExternalTileset(
+      externalFileName,
+      externalFileBuffer
+    );
+    const externalRoot = externalTileset.root;
+
+    // All relevant properties of the tile are replaced with
+    // the properties of the external tileset root
+    tile.geometricError = externalRoot.geometricError;
+    tile.content = externalRoot.content;
+    tile.contents = externalRoot.contents;
+    tile.children = externalRoot.children;
+    tile.boundingVolume = externalRoot.boundingVolume;
+    tile.transform = externalRoot.transform;
+  }
+
+  /**
+   * This is called for each tile of the source tileset that has multiple contents.
+   *
+   * It will process each content:
+   *
+   * If the content points to an external tileset, then
+   * - The external tileset is 'combined' by calling `processExternalTileset`
+   * - The root of this (combined) external tileset is added as a child to the give tile
+   *
+   * Otherwise, the URL of the content is updated to be relative to
+   * the root of the resulting combined tileset.
+   *
+   * @param currentDirectory - The current directory (see `combineTilesetsInternal`)
+   * @param tile - The current tile
+   */
+  private async combineMultipleContentsInternal(
+    currentDirectory: string,
+    tile: Tile
+  ): Promise<void> {
+    if (!this.tilesetSource || !this.tilesetTarget) {
+      throw new DeveloperError("The source and target must be defined");
+    }
+    const contents = tile.contents;
+    if (!contents) {
+      throw new DeveloperError(
+        "This should only be called for tiles with multiple contents"
+      );
+    }
+
+    // The new contents (omitting the external tilesets),
+    // and the new children (containing tiles for external tileset roots)
+    const newContents: Content[] = [];
+    let newChildren: Tile[] | undefined = undefined;
+
+    for (const content of contents) {
+      // Obtain the data of the content, and determine whether it is
+      // an external tileset
+      const contentUri = TilesetCombiner.obtainContentUri(content);
+      const externalFileName = Paths.join(currentDirectory, contentUri);
+      const externalFileBuffer = this.tilesetSource.getValue(externalFileName);
+      if (!externalFileBuffer) {
+        throw new TilesetError(`No data found for ${externalFileName}`);
       }
-      const externalTileset = JSON.parse(
-        externalTilesetBuffer.toString()
-      ) as Tileset;
-      await this.combineTilesetsInternal(
-        externalTilesetDirectory,
-        externalTileset,
-        tile
+      const contentData = new BufferedContentData(
+        contentUri,
+        externalFileBuffer
       );
+      const isTileset = await this.externalTilesetDetector(contentData);
+
+      if (!isTileset) {
+        // When the data is not an external tileset, then just update
+        // the content URI to point to the path that the content data
+        // will end up in
+        const newUri = Paths.relativize(".", externalFileName);
+        content.uri = newUri;
+        newContents.push(content);
+        continue;
+      }
+
+      // The data is an external tileset. Process (combine) this
+      // tileset, recursively
+      const externalTileset = await this.processExternalTileset(
+        externalFileName,
+        externalFileBuffer
+      );
+      const externalRoot = externalTileset.root;
+
+      // Add a tile (that has the properties from the external tileset
+      // root) as a new child
+      const newChild: Tile = {
+        geometricError: externalRoot.geometricError,
+        content: externalRoot.content,
+        contents: externalRoot.contents,
+        children: externalRoot.children,
+        boundingVolume: externalRoot.boundingVolume,
+        transform: externalRoot.transform,
+      };
+      if (newChildren === undefined) {
+        newChildren = [newChild];
+      } else {
+        newChildren.push(newChild);
+      }
     }
+    Tiles.setContents(tile, newContents);
+    tile.children = newChildren;
+  }
+
+  /**
+   * Returns the URI of the given content, handling the case that it
+   * might be stored as the (legacy) `url` property.
+   *
+   * If the given content contains the (legacy) `url` property, then
+   * it will be updated, in place: The `uri` property will be set to
+   * the value of the `url` property, and the `url` property will
+   * be deleted.
+   *
+   * @param content - The `Content`
+   * @returns The content URI
+   */
+  private static obtainContentUri(content: Content): string {
+    let contentUri = content.uri;
+    if (contentUri !== undefined) {
+      return contentUri;
+    }
+    // This is the case for legacy data (including some of the
+    // original spec data), so handle this case explicitly here.
+    const legacyContent = content as any;
+    if (legacyContent.url === undefined) {
+      // This should never be the case:
+      throw new TilesetError(
+        "Content does neither contain a 'uri' nor a (legacy) 'url' property"
+      );
+    }
+    // Remove the legacy content.url, and set the URL
+    // as the content.uri instead
+    logger.warn(
+      "The 'url' property of tile content is deprecated. Using 'uri' in combined result."
+    );
+    contentUri = legacyContent.url;
+    delete legacyContent.url;
+    legacyContent.uri = contentUri;
+    return contentUri;
   }
 
   /**