diff --git a/doc/source/user_manual/modules/mwl.rst b/doc/source/user_manual/modules/mwl.rst
index 99bd6d2..d4df56a 100644
--- a/doc/source/user_manual/modules/mwl.rst
+++ b/doc/source/user_manual/modules/mwl.rst
@@ -2,3 +2,21 @@
 
 Multi-wavelength interface
 --------------------------
+
+Overview
+~~~~~~~~
+
+The multi-wavelength module provides an interface for spectral energy 
+distributions (SEDs). The SED defines the observed photon flux as function 
+of energy (or frequency) from a particular source, and this information 
+can be used in a fit to determine spectral source parameters.
+
+:ref:`fig_uml_mwl` present an overview over the C++ classes of the 
+multi-wavelength module and their relations.
+
+.. _fig_uml_mwl:
+
+.. figure:: uml_mwl.png
+   :width: 40%
+
+   Multi-wavelength module
diff --git a/doc/source/user_manual/modules/uml_mwl.png b/doc/source/user_manual/modules/uml_mwl.png
new file mode 100644
index 0000000..ed03470
--- /dev/null
+++ b/doc/source/user_manual/modules/uml_mwl.png