This paper introduces the applications of Computer Assisted Orthopaedic Surgery (CAOS), including pre-operative planner, surgical simulator, intra-operative navigation systems and medical robotics. Basic principals of surgical navigation are also discussed. In view of the high precision requirement for navigation assisted procedures, there exists a definite need for development of medical robotics to carry out these procedures. We have a brief review on the different robot systems and present the surgical navigation arm developed in the Chinese University of Hong Kong. The laboratory test results of navigation guided distal targeting of intramedullary nail with or without surgical navigation arm showed the better repeatability and precision when the robot arm was used. So far the surgical navigation arm has been successfully applied in clinical practice, including distal targeting of intramedullary nailing; percutaneous sacro-iliac screw and percutaneous trans-iliac screw in pelvic fractures and hip screw fixation for fracture of femoral neck. To conclude, medical robotics is an essential component of the future CAOS.