diff --git a/README.md b/README.md
index e8ca942a..119d4781 100644
--- a/README.md
+++ b/README.md
@@ -11,10 +11,28 @@ Join us
 
 - http://launchpad.net/taskflow
 
-Help
-----
+Testing and requirements
+------------------------
 
-**Tox.ini**
+### Requirements
+
+Because TaskFlow has many optional (pluggable) parts like persistence
+backends and engines, we decided to split our requirements into two
+parts:
+- things that are absolutely required by TaskFlow (you can't use
+  TaskFlow without them) are put to `requirements.txt`;
+- things that are required by some optional part of TaskFlow (you
+  can use TaskFlow without them) are put to `optional-requirements.txt`;
+  if you want to use the feature in question, you should add that
+  requirements to your project or environment;
+- as usual, things that required only for running tests are put
+  to `test-requirements.txt`.
+
+### Tox.ini
+
+Our tox.ini describes several test environments that allow to test
+TaskFlow with different python versions and sets of requirements
+installed.
 
 To generate tox.ini, use the `toxgen.py` script by first installing
 [toxgen](https://pypi.python.org/pypi/toxgen/) and then provide that script
diff --git a/optional-requirements.txt b/optional-requirements.txt
new file mode 100644
index 00000000..4c0d387e
--- /dev/null
+++ b/optional-requirements.txt
@@ -0,0 +1,22 @@
+# This file lists dependencies that are used by different
+# pluggable (optional) parts of TaskFlow, like engines
+# or persistence backends. They are not strictly required
+# by TaskFlow (you can use TaskFlow without them), but
+# so they don't go to requirements.txt.
+
+# Database (sqlalchemy) persistence:
+SQLAlchemy<=0.7.99,<=0.9.99
+alembic>=0.4.1
+
+# Database (sqlalchemy) persistence with MySQL:
+pyMySQL
+MySQL-python
+
+# Database (sqlalchemy) persistence with PostgreSQL:
+psycopg2
+
+# ZooKeeper backends
+kazoo>=1.3.1
+
+# Eventlet may be used with parallel engine
+eventlet>=0.13.0
diff --git a/tox-tmpl.ini b/tox-tmpl.ini
index 1ad399a6..d8d0a1d1 100644
--- a/tox-tmpl.ini
+++ b/tox-tmpl.ini
@@ -34,7 +34,7 @@ commands = pylint
 
 [testenv:cover]
 basepython = python2.7
-deps = {[testenv:py27-sa9-mysql-ev]deps}
+deps = {[testenv:py27]deps}
 commands = python setup.py testr --coverage --testr-args='{posargs}'
 
 [testenv:venv]
@@ -52,7 +52,9 @@ deps = {[testenv:py26-sa7-mysql-ev]deps}
 
 [testenv:py27]
 basepython = python2.7
-deps = {[testenv:py27-sa9-mysql-ev]deps}
+deps = -r{toxinidir}/requirements.txt
+       -r{toxinidir}/optional-requirements.txt
+       -r{toxinidir}/test-requirements.txt
 
 [testenv:py33]
 basepython = python3.3
diff --git a/tox.ini b/tox.ini
index 13bbf71f..e30b39ec 100644
--- a/tox.ini
+++ b/tox.ini
@@ -65,7 +65,7 @@ commands = pylint
 
 [testenv:cover]
 basepython = python2.7
-deps = {[testenv:py27-sa9-mysql-ev]deps}
+deps = {[testenv:py27]deps}
 commands = python setup.py testr --coverage --testr-args='{posargs}'
 
 [testenv:venv]
@@ -81,7 +81,9 @@ deps = {[testenv:py26-sa7-mysql-ev]deps}
 
 [testenv:py27]
 basepython = python2.7
-deps = {[testenv:py27-sa9-mysql-ev]deps}
+deps = -r{toxinidir}/requirements.txt
+    -r{toxinidir}/optional-requirements.txt
+    -r{toxinidir}/test-requirements.txt
 
 [testenv:py33]
 basepython = python3.3