Part 1 of the volume presents biomechanical models that have been developed in a CAS context and used during surgery. This is particularly new since most of the soft tissues models already proposed concern Computer Assisted Planning, with a pre-operative use of the models. Then, the volume addresses the two key issues raised for an intra-operative use of soft tissues models, namely (Part 2) “how to estimate the in vivo mechanical behavior of the tissues?” (i.e. what are the values of the mechanical parameters that can deliver realistic patient-specific behavior?) and (Part 3) “how to build a modeling platform that provides generic real-time (or at least interactive-time) numerical simulations?”