diff --git a/tox.ini b/tox.ini
index 19080a648..440004177 100644
--- a/tox.ini
+++ b/tox.ini
@@ -82,15 +82,18 @@ deps =
 
     mock
     lxml
+    coverage
 commands =
     /usr/bin/find "{toxinidir}" -name '*.pyc' -delete
     # Make all greater-thans equals so we test the oldest version of our direct
     # dependencies, but make the pyopenssl 17.0, which can work against an
     # OpenSSL 1.1 compiled cryptography (as older ones don't compile on Travis).
     /bin/sh -c 'python -m synapse.python_dependencies | sed -e "s/>=/==/g" -e "s/psycopg2==2.6//" -e "s/pyopenssl==16.0.0/pyopenssl==17.0.0/" | xargs pip install'
-    # Install Synapse itself. This won't update any libraries.
-    pip install -e .
-    {envbindir}/trial {env:TRIAL_FLAGS:} {posargs:tests} {env:TOXSUFFIX:}
+
+    # Add this so that coverage will run on subprocesses
+    /bin/sh -c 'echo "import coverage; coverage.process_startup()" > {envsitepackagesdir}/../sitecustomize.py'
+
+    {envbindir}/coverage run "{envbindir}/trial"  {env:TRIAL_FLAGS:} {posargs:tests} {env:TOXSUFFIX:}
 
 [testenv:packaging]
 skip_install=True