diff --git a/Cabal/tests/PackageTests.hs b/Cabal/tests/PackageTests.hs
index 69b6c8000eb..7cec4381ae0 100644
--- a/Cabal/tests/PackageTests.hs
+++ b/Cabal/tests/PackageTests.hs
@@ -9,7 +9,8 @@ module Main where
 import Data.Version (Version(Version))
 import Distribution.Simple.Utils (cabalVersion)
 import Distribution.Text (display)
-import System.Directory (setCurrentDirectory)
+import System.Directory (getCurrentDirectory, setCurrentDirectory)
+import System.FilePath ((</>))
 import Test.Framework (Test, TestName, defaultMain, testGroup)
 import Test.Framework.Providers.HUnit (hUnitTestToTests)
 import qualified Test.HUnit as HUnit
@@ -29,7 +30,7 @@ import PackageTests.BuildDeps.TargetSpecificDeps1.Check
 import PackageTests.BuildDeps.TargetSpecificDeps2.Check
 import PackageTests.BuildDeps.TargetSpecificDeps3.Check
 import PackageTests.BuildTestSuiteDetailedV09.Check
-import PackageTests.PackageTester (compileSetup)
+import PackageTests.PackageTester (PackageSpec(..), compileSetup)
 import PackageTests.PathsModule.Executable.Check
 import PackageTests.PathsModule.Library.Check
 import PackageTests.PreProcess.Check
@@ -43,8 +44,8 @@ import PackageTests.TestSuiteExeV10.Check
 hunit :: TestName -> HUnit.Test -> Test
 hunit name test = testGroup name $ hUnitTestToTests test
 
-tests :: Version -> [Test]
-tests version =
+tests :: Version -> PackageSpec -> [Test]
+tests version inplaceSpec =
     [ hunit "BuildDeps/SameDepsAllRound"
       PackageTests.BuildDeps.SameDepsAllRound.Check.suite
       -- The two following tests were disabled by Johan Tibell as
@@ -79,7 +80,7 @@ tests version =
     , hunit "EmptyLib/emptyLib"
       PackageTests.EmptyLib.Check.emptyLib
     , hunit "BuildTestSuiteDetailedV09"
-      PackageTests.BuildTestSuiteDetailedV09.Check.suite
+      $ PackageTests.BuildTestSuiteDetailedV09.Check.suite inplaceSpec
     ] ++
     -- These tests are only required to pass on cabal version >= 1.7
     (if version >= Version [1, 7] []
@@ -104,9 +105,17 @@ tests version =
 
 main :: IO ()
 main = do
+    wd <- getCurrentDirectory
+    let dbFile = wd </> "dist/package.conf.inplace"
+        inplaceSpec = PackageSpec
+            { directory = []
+            , configOpts = [ "--package-db=" ++ dbFile
+                           , "--constraint=Cabal == " ++ display cabalVersion
+                           ]
+            }
     putStrLn $ "Cabal test suite - testing cabal version " ++
         display cabalVersion
     setCurrentDirectory "tests"
     -- Create a shared Setup executable to speed up Simple tests
     compileSetup "."
-    defaultMain (tests cabalVersion)
+    defaultMain (tests cabalVersion inplaceSpec)
diff --git a/Cabal/tests/PackageTests/BuildTestSuiteDetailedV09/Check.hs b/Cabal/tests/PackageTests/BuildTestSuiteDetailedV09/Check.hs
index f23b0ac0558..33214d67eab 100644
--- a/Cabal/tests/PackageTests/BuildTestSuiteDetailedV09/Check.hs
+++ b/Cabal/tests/PackageTests/BuildTestSuiteDetailedV09/Check.hs
@@ -5,10 +5,13 @@ import System.FilePath ((</>))
 
 import PackageTests.PackageTester
 
-suite :: Test
-suite = TestCase $ do
+suite :: PackageSpec -> Test
+suite inplaceSpec = TestCase $ do
     let dir = "PackageTests" </> "BuildTestSuiteDetailedV09"
-        spec = PackageSpec dir ["--enable-tests"]
+        spec = inplaceSpec
+            { directory = dir
+            , configOpts = "--enable-tests" : configOpts inplaceSpec
+            }
     confResult <- cabal_configure spec
     assertEqual "configure failed!" (successful confResult) True
     buildResult <- cabal_build spec
diff --git a/Cabal/tests/README b/Cabal/tests/README
new file mode 100644
index 00000000000..5b4481468eb
--- /dev/null
+++ b/Cabal/tests/README
@@ -0,0 +1,22 @@
+Writing Package Tests
+=====================
+
+The tests under the `PackageTests` directory define and build packages which
+exercise various components of Cabal. Each test case is an `HUnit` test. The
+entry point for the test suite, where all the test cases are listed, is
+`PackageTests.hs`. There are utilites for calling the stages of Cabal's build
+process in `PackageTests/PackageTester.hs`; have a look at an existing test case
+to see how they're used.
+
+It is very important that package tests use the in-place version of Cabal,
+rather than the system version. Several long-standing bugs in the test suite
+were caused by testing the system (rather than the newly-compiled) version of
+Cabal. There are two places where the system Cabal can accidentally be invoked:
+1. Compiling `Setup.hs`. `runghc` needs to be told about the in-place package
+database. This issue should be solved for all future package tests; see
+`compileSetup` in `PackageTests/PackageTester.hs`.
+2. Compiling a package which depends on Cabal. In particular, packages with
+`detailed` type test suites depend on the Cabal library directly, so it is
+important that they are configured to use the in-place package database. The
+test suite already creates a stub `PackageSpec` for this case; see
+`PackageTests/BuildTestSuiteDetailedV09/Check.hs` to see how it is used.