OpenWalnut is an open source tool for multi-modal medical and brain data visualization. Its universality allows it to be easily extended and used in a large variety of application cases. It is both, a tool for the scientific user and a powerful framework for the visualization researcher. OpenWalnut is licensed under the terms of the GNU Lesser General Public License version 3. Written entirely in Standard C++ and using a number of portable libraries (e.g. Qt, Boost and OpenSceneGraph) it runs on common GNU/Linux operating systems, macOS, and Windows. For more information, including system requirements, see our Wiki.