diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index f2a912b5664..a7102bef96e 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -16,6 +16,8 @@ Thanks for your interest in contributing to Lodestar. It's people like you that
 
 ## Tests
 
+To run tests:
+
 - :test_tube: Run `lerna run test:unit` for unit tests.
 - :test_tube: Run `lerna run test:e2e` for end-to-end tests.
 - :test_tube: Run `lerna run test:spec` for spec tests.
@@ -23,6 +25,13 @@ Thanks for your interest in contributing to Lodestar. It's people like you that
 - :test_tube: Run `yarn check-types` to check TypeScript types.
 - :test_tube: Run `yarn lint` to run the linter (ESLint).
 
+Contributing to tests:
+
+- Test must not depend on external live resources, such that running tests for a commit must be deterministic:
+  - Do not pull data from external APIs like execution JSON RPC (instead run a local node).
+  - Do not pull unpinned versions from dockerhub (use deterministic tag) or Github (checkout commit not branch).
+  - Carefully design tests that depend on timing sensitive events like p2p network e2e tests. Consider that Github runners are significantly less powerful than your development environment.
+
 ### Debugging Spec Tests
 
 - To fix errors always focus on passing all minimal tests first without running mainnet tests.