Passive and semi-active aids ince 1985, a team of computer scien-surgeons has been involved in a project at Grenoble Hospital called computer assisted medical interventions (CAMI). The aim of the project is to help surgeons and physicians use multimodal data in a rational and quantitative way in order to plan and to perform medical interventions. Recent advances in medical imaging systems such as CT and MRI have stimulated research on the interpretation of medical images. Nevertheless, very few systems allow for an efficient therapeutic use of the wealth of information these images contain , which is CAMI's twofold objective: =Define an operative strategy that takes advantage of the localizing capabilities of imaging, and make this strategy available in an operative reference system. This goal requires models and processing of basic data, in conjunction with a priori knowledge , in order to define an optimal strategy. =Perform the previously defined operative strategy, with the aid of a suitable guidance system, under appropriate imaging supervision. Without help, executing a strategy can sometimes be very difficult. First, reproducing a defined strategy directly raises problems, for one has to mentally match geometrical information observed in different reference systems (mainly intra-operative scenes or images with pre-operative images). Then, complex interventions may be necessary on organs difficult to reach or for which a human operator will only have poor or no visibility. In addition, surgical tools, such as probes, most often have to be very precisely positioned, and submillimetric accuracy may be required for microsurgery. Finally, some interventions may be dangerous for the medical staff (e.g., contamination or irradiation). In all these instances, optical or mechanical guidance systems are required. The ultimate success of this research hinges on complex robotics systems and their various sensors. The aforementioned objectives aim at improving the quality of the interventions by making it easier, more accurate, closer to a a pre-operative simulation where accurate objectives can be defined, and sometimes faster. Also, it may be possible to devise new interventions and to validate protocols of therapeutic research. Obviously , this is long term research, with many potential clinical applications. Yet, a general methodology can be applied to various clinical situations [ 11, as presented below. In order to understand the methodology and applications of CAMI, it is necessary to be aware of current technology. Many issues in the CAMI project are concerned with geometrical localization problems. Hardware issues mainly refer …