The Insight Toolkit (ITK) is an open-source software package for medical image processing, segmentation and registration. ITK was developed by a world-wide consortium of academic and industry groups under contract to the National Library of Medicine. After a brief introduction to ITK, the talk will describe the unique software and social processes that make ITK a significant and permanent resource for medical researchers. This will be a critical review of the project that will provide valuable lessons for future distributed software developments.