diff --git a/test/README.md b/test/README.md
index cc7f1fd..49958cc 100644
--- a/test/README.md
+++ b/test/README.md
@@ -1,3 +1,11 @@
 # diffuser Test Suite Layout
 
-Placeholder structure (diffuser currently has no runtime module files).
+`test/{basic,integration,edge}/` — standard EVOLV three-tier test
+structure. The diffuser runtime modules live at `src/nodeClass.js`,
+`src/specificClass.js`, and `src/commands/`; they implement the OTR /
+ΔP aeration model and the `data.airFlow` / `set.kla` topic handlers
+documented in `CONTRACT.md`.
+
+Helpers shared across tiers belong in `test/helpers/`.
+
+Run: `node --test test/basic test/integration test/edge`.