diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -4,3 +4,14 @@
 Search service for the Software Heritage archive.
 
 Currently uses ElasticSearch, and provides only origin search (by URL and metadata)
+
+# Dependencies
+
+Python tests for this module include tests that cannot be run without a local
+ElasticSearch instance, so you need the ElasticSearch server executable on your
+machine (no need to have a running ElasticSearch server). On a Debian-like
+host:
+
+```
+sudo apt install elasticsearch
+```