The two-volume set LNCS 2878 and LNCS 2879 constitute the refereed proceedings of the 6th International Conference on Medical Image Computing and Computer-Assisted Intervention, MICCAI 2003, held in Montreal, Canada in November 2003.The 206 revised full papers and 25 revised short papers presented were carefully reviewed and selected from a total of 599 submissions. The two-volume proceedings offers topical sections on simulation and planning, robotic mechanisms and mechanical properties of tissue, interventional registration, cardiac imaging, segmentation, clinical applications of medical image computing, medical image processing, visualization and navigation, interventional imaging, image morphometry, and registration and atlases.