diff --git a/xos/README.md b/xos/README.md
new file mode 100644
index 0000000..d761d59
--- /dev/null
+++ b/xos/README.md
@@ -0,0 +1,21 @@
+## XOS Source Tree
+
+This is the main directory for XOS. Sub-directories include:
+
+* admin_customize, templates -- related to Django GUI
+* configurations -- collection of canned configurations
+* core -- core model definitions
+* generators -- tools to generate auxiliary structures from data model
+* model_policies -- invariants on the data model
+* nginx, uwsgi -- related to web server that runs XOS
+* openstack -- client-side interaction with OpenStack (to be depreciated)
+* services -- model definitions for a set of services
+* synchronizers -- collection of synchronizers
+* test -- system-wide tests to be collected here
+* tosca -- tosca modeling layer on top of RESTful API
+* tools -- assorted tools and scripts
+* xos -- common source code for all Django applications
+* xos_configuration -- top-level XOS configuration parameters
+
+Of these, configuration, services, and synchronizers are most
+relevant to developers.