Voreen is an open source rapid application development framework for the interactive visualization and analysis of multi-modal volumetric data sets. It provides GPU-based volume rendering and data analysis techniques and offers high flexibility when developing new analysis workflows in collaboration with domain experts. The Voreen framework consists of a multi-platform C++ library, which can be easily integrated into existing applications, and a Qt-based stand-alone application. It is licensed under the terms of the GNU General Public License. The Voreen project has been initiated and is maintained by the Visualization & Computer Graphics Research Group at the University of Münster as part of the collaborative research center SFB 656 MoBil (Project Z1, Project Ö).