diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 9fbb746..b46e412 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -37,6 +37,8 @@
 
 2. Download the source code for x/pkgsite:
    `git clone https://go.googlesource.com/pkgsite`
+   NOTE: You will need Go 1.18 (as of this writing, the beta2 release) to
+   develop pkgsite.
 
 3. Review the [design document](doc/design.md).
 
diff --git a/README.md b/README.md
index 35bb78d..8fee08c 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,11 @@
 
 You can check it out at [https://pkg.go.dev](https://pkg.go.dev).
 
+## Requirements
+
+Pkgsite requires Go 1.18 to run. The last commit that works with older versions
+of Go is 4d836c6a652cde92f433967680dfd6171a91ec12.
+
 ## Issues
 
 If you want to report a bug or have a feature suggestion, please first check