diff --git a/packages/localize/CHANGELOG.md b/packages/localize/CHANGELOG.md
index 64fd07d88e..969dfc657b 100644
--- a/packages/localize/CHANGELOG.md
+++ b/packages/localize/CHANGELOG.md
@@ -26,6 +26,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - It is no longer necessary to provide a message `id`. When omitted, an id will
   be automatically generated from the string contents of the template.
 
+### Fixed
+
+- `// msgdesc` descriptions are now correctly emitted as XLIFF `<note>`
+  elements, instead of crashing.
+
 ## [0.5.1] - 2020-11-09
 
 ### Fixed
diff --git a/packages/localize/README.md b/packages/localize/README.md
index 1ebc1b400f..72b7bd6661 100644
--- a/packages/localize/README.md
+++ b/packages/localize/README.md
@@ -12,7 +12,7 @@
 
 <img src="./rgb_lit.png" width="150" height="100" align="right"></img>
 
-###### [Features](#features) | [Overview](#overview) | [Modes](#modes) | [Tutorial](#tutorial) | [API](#api) | [Status event](#lit-localize-status-event) | [Localized mixin](#localized-mixin) | [CLI](#cli) | [Config file](#config-file) | [FAQ](#faq)
+###### [Features](#features) | [Overview](#overview) | [Modes](#modes) | [Tutorial](#tutorial) | [API](#api) | [Descriptions](#descriptions) | [Status event](#lit-localize-status-event) | [Localized mixin](#localized-mixin) | [CLI](#cli) | [Config file](#config-file) | [FAQ](#faq)
 
 > @lit/localize is a library and command-line tool for localizing web
 > applications that are based on lit-html and LitElement.
@@ -376,6 +376,49 @@ html`Hola <b>${getUsername()}!</b>`;
 
 Name of the [`lit-localize-status`](#lit-localize-status-event) event.
 
+## Descriptions
+
+You can add descriptions to messages using special `// msgdesc:` comments.
+Message descriptions help translators understand the context of each string they
+are translating.
+
+```ts
+// msgdesc: Greeting to everybody on homepage
+msg(html`Hello <b>World</b>!`);
+```
+
+Descriptions are represented in XLIFF using `<note>` elements.
+
+```xml
+<trans-unit id="0h3c44aff2d5f5ef6b">
+  <note>Greeting to everybody on homepage</note>
+  <source>Hello <ph id="0">&lt;b></ph>World<ph id="1">&lt;/b></ph>!</source>
+</trans-unit
+```
+
+You can also apply a `// msgdesc:` comment to a class, function, or block, in
+which case the description will apply recursively to all `msg` calls within it.
+If there are multiple descriptions that apply to a `msg` call, then they are
+concatenated with a forward-slash in top-down order. This can be useful for
+describing the context of an entire group of messages.
+
+```ts
+// msgdesc: Homepage
+class MyHomepage extends Localized(LitElement) {
+  render() {
+    // msgdesc: Greeting to everybody
+    return msg(html`Hello <b>World</b>!`);
+  }
+}
+```
+
+```xml
+<trans-unit id="0h3c44aff2d5f5ef6b">
+  <note>Homepage / Greeting to everybody</note>
+  <source>Hello <ph id="0">&lt;b></ph>World<ph id="1">&lt;/b></ph>!</source>
+</trans-unit>
+```
+
 ## `lit-localize-status` event
 
 In runtime mode, whenever a locale change starts, finishes successfully, or
diff --git a/packages/localize/src/formatters/xliff.ts b/packages/localize/src/formatters/xliff.ts
index e00d19a225..19d6798f51 100644
--- a/packages/localize/src/formatters/xliff.ts
+++ b/packages/localize/src/formatters/xliff.ts
@@ -250,9 +250,10 @@ export class XliffFormatter implements Formatter {
 
       if (descStack.length > 0) {
         // https://docs.oasis-open.org/xliff/v1.2/os/xliff-core.html#note
-        const note = document.createElement('note');
+        const note = doc.createElement('note');
         note.appendChild(doc.createTextNode(descStack.join(' / ')));
         transUnit.appendChild(note);
+        indent(transUnit, 1);
       }
 
       // https://docs.oasis-open.org/xliff/v1.2/os/xliff-core.html#source
diff --git a/packages/localize/testdata/xliff/goldens/foo.ts b/packages/localize/testdata/xliff/goldens/foo.ts
index 13a69da59d..62a7b06dd6 100644
--- a/packages/localize/testdata/xliff/goldens/foo.ts
+++ b/packages/localize/testdata/xliff/goldens/foo.ts
@@ -31,3 +31,14 @@ msg(`Hello World!`);
 msg((name) => `Hello ${name}!`, {args: ['Friend']});
 msg(html`Hello <b>World</b>!`);
 msg((name) => html`Hello <b>${name}</b>!`, {args: ['Friend']});
+
+// msgdesc: Description of 0
+msg('described 0');
+
+// msgdesc: Parent description
+export function described() {
+  // msgdesc: Description of 1
+  msg('described 1');
+  // msgdesc: Description of 2
+  msg('described 2');
+}
diff --git a/packages/localize/testdata/xliff/goldens/tsout/es-419.ts b/packages/localize/testdata/xliff/goldens/tsout/es-419.ts
index 7104cdb9d1..2fedfc1f06 100644
--- a/packages/localize/testdata/xliff/goldens/tsout/es-419.ts
+++ b/packages/localize/testdata/xliff/goldens/tsout/es-419.ts
@@ -23,4 +23,7 @@ export const templates = {
     <b>${x}</b>
     <i>y</i>
     <b>${x}</b>`,
+  as03c68d79ad36e8d4: `described 0`,
+  as03c68e79ad36ea87: `described 1`,
+  as03c68f79ad36ec3a: `described 2`,
 };
diff --git a/packages/localize/testdata/xliff/goldens/tsout/zh_CN.ts b/packages/localize/testdata/xliff/goldens/tsout/zh_CN.ts
index 66d8e9c9bc..9022de1fb1 100644
--- a/packages/localize/testdata/xliff/goldens/tsout/zh_CN.ts
+++ b/packages/localize/testdata/xliff/goldens/tsout/zh_CN.ts
@@ -23,4 +23,7 @@ export const templates = {
   as00ad08ebae1e0f74: (name: any) => `Hello ${name}!`,
   ah3c44aff2d5f5ef6b: html`Hello <b>World</b>!`,
   ah82ccc38d4d46eaa9: (name: any) => html`Hello <b>${name}</b>!`,
+  as03c68d79ad36e8d4: `described 0`,
+  as03c68e79ad36ea87: `described 1`,
+  as03c68f79ad36ec3a: `described 2`,
 };
diff --git a/packages/localize/testdata/xliff/goldens/xliff/es-419.xlf b/packages/localize/testdata/xliff/goldens/xliff/es-419.xlf
index cf512cd16e..a5546333a3 100644
--- a/packages/localize/testdata/xliff/goldens/xliff/es-419.xlf
+++ b/packages/localize/testdata/xliff/goldens/xliff/es-419.xlf
@@ -48,6 +48,18 @@
   <source>Hello <ph id="0">&lt;b>${name}&lt;/b></ph>!</source>
   <target>Hola <ph id="0">&lt;b>${name}&lt;/b></ph>!</target>
 </trans-unit>
+<trans-unit id="as03c68d79ad36e8d4">
+  <note>Description of 0</note>
+  <source>described 0</source>
+</trans-unit>
+<trans-unit id="as03c68e79ad36ea87">
+  <note>Parent description / Description of 1</note>
+  <source>described 1</source>
+</trans-unit>
+<trans-unit id="as03c68f79ad36ec3a">
+  <note>Parent description / Description of 2</note>
+  <source>described 2</source>
+</trans-unit>
 </body>
 </file>
 </xliff>
diff --git a/packages/localize/testdata/xliff/goldens/xliff/zh_CN.xlf b/packages/localize/testdata/xliff/goldens/xliff/zh_CN.xlf
index 1ed55b9b0a..e7a4abc5cc 100644
--- a/packages/localize/testdata/xliff/goldens/xliff/zh_CN.xlf
+++ b/packages/localize/testdata/xliff/goldens/xliff/zh_CN.xlf
@@ -39,6 +39,18 @@
 <trans-unit id="ah82ccc38d4d46eaa9">
   <source>Hello <ph id="0">&lt;b>${name}&lt;/b></ph>!</source>
 </trans-unit>
+<trans-unit id="as03c68d79ad36e8d4">
+  <note>Description of 0</note>
+  <source>described 0</source>
+</trans-unit>
+<trans-unit id="as03c68e79ad36ea87">
+  <note>Parent description / Description of 1</note>
+  <source>described 1</source>
+</trans-unit>
+<trans-unit id="as03c68f79ad36ec3a">
+  <note>Parent description / Description of 2</note>
+  <source>described 2</source>
+</trans-unit>
 </body>
 </file>
 </xliff>
diff --git a/packages/localize/testdata/xliff/input/foo.ts b/packages/localize/testdata/xliff/input/foo.ts
index 13a69da59d..62a7b06dd6 100644
--- a/packages/localize/testdata/xliff/input/foo.ts
+++ b/packages/localize/testdata/xliff/input/foo.ts
@@ -31,3 +31,14 @@ msg(`Hello World!`);
 msg((name) => `Hello ${name}!`, {args: ['Friend']});
 msg(html`Hello <b>World</b>!`);
 msg((name) => html`Hello <b>${name}</b>!`, {args: ['Friend']});
+
+// msgdesc: Description of 0
+msg('described 0');
+
+// msgdesc: Parent description
+export function described() {
+  // msgdesc: Description of 1
+  msg('described 1');
+  // msgdesc: Description of 2
+  msg('described 2');
+}