Computer assisted surgery (CAS) attempts primarily to optimize the performance of medical tasks. This consists in planning optimal strategies and accurately transferring and executing these plans in intra-operative conditions. One clinical purpose is to minimize the intervention invasiveness by increasing the selectivity of anatomical target localization and reducing the aggressiveness of the target access through minimal incisions or natural openings. This generally results for the surgeon in a lack of perception (vision, tactile feeling) and dexterity. It may also raise accessibility problems and may require a higher positional and force accuracy. Therefore, an interface device of some sort is needed in order to connect the "information world" of images, plans, and computers, to the physical world of surgeons, patients, and tools. Such a device is called a "guiding system". We discuss the PADyC system whose purpose is to limit the possible actions of the surgeon to what is strictly authorized in the planning. PADyC relies on man-machine cooperation. We present its principles and we discuss its application to cardiac punctures.