The use of robotics and navigated systems to prepare, perform and reinforce surgical interventions is described by the term "computer aided surgery" (CAS). CAS is expected to make surgery even more precise, safer and cheaper. It comprehends computer based supporting systems (e. g. therapy planning, simulation, navigation), assisting systems, programmable automates and the so called master-slave-systems. When introducing technical innovations, the potential benefit of the patient is the essential issue. Then only practicability under clinical conditions, and additional aspects can be discussed. Conclusively, less spectacular applications of CAS like computer based supporting or assisting systems are more relevant for current practical use than visionary robotic systems.