The advent of medical imaging and new operating techniques has revolutionized the working methods of medical professionals. This change requires physicians and surgeons to undergo additional training. Hence, the development of appropriate tools, like medical simulators, is of paramount importance. Within this context, this book focuses on physical modeling of soft tissue and collision detection in virtual environments. First, we present various physical models and numerical resolution methods associated with deformable objects. We then propose an alternative model developed for soft tissue simulation. This model, based on Pascal's principle, allows us in a relatively simple way to represent soft tissue. Next, we present various existing algorithms for collision detection, as well as the difficulty in adapting these algorithms in medical simulators. We then present the algorithms developed in our work to deal with this problem. These algorithms have better numerical robustness and are optimized. We apply these methods within the framework of an echographic simulator of the human thigh and a simulator for the arthroscopic reconstruction of the anterior cruciate ligament of the knee.