MITK stands for Medical Imaging ToolKit. It is a C++ library for integrated medical image processing and analyzing developed by the Medical Image Processing Group, Key Laboratory of Complex Systems and Intelligence Science, Institute of Automation, the Chinese Academy of Sciences. The development of MITK is inspired by the big success of open source softwares VTK and ITK, and its main purpose is to provide medical image community a consistent framework to combine the function of medical image segmentation, registration and visualization. Much as the style of VTK, MITK uses the traditional Object-Oriented design methods, and doesn't use the Generic Programming style used by ITK. So the syntax and interface of MITK is simple and intuitive. Now MITK is a free software and can be used freely for research and education purpose. We hope that MITK will become another available choice for the medical imaging community.