diff --git a/README.md b/README.md
index 4117de8..cfef3c4 100644
--- a/README.md
+++ b/README.md
@@ -43,6 +43,18 @@ md_tools.sh
 saml_tools.sh
 ```
 
+## Environment
+
+Besides `BIN_DIR` and `LIB_DIR` above, the following environment variables are required:
+
+| Variable | |
+| --- | --- |
+| `CACHE_DIR` | A persistent HTTP cache |
+| `TMPDIR` | A temporary directory |
+| `LOG_FILE` | A persistent log file |
+
+Some OSes define `TMPDIR` and some do not. In any case, a temporary directory is required to use these scripts.
+
 ## Overview
 
 ### `cget.sh`