This paper describes the development of a system for simultaneous planning and execution of surgical operations in the cranio-maxillo facial area. Simultaneous planning and execution is the process of taking an implicit task description, planning a sequence of explicit execution commands e.g. for robots and monitoring their execution. As the execution planning process is running completely on-line, that means during the actual execution of the assembly task, the planning process is highly reactive based on sensor information about the robot’s present environment. In order to meet the problem that medical data is usually complex and needs time-costly preprocessing an appropriate architecture for evaluating sensor data has been developed. In this paper, a detailed presentation of the phases of execution planning and sensor data evaluation is given. As an example, the execution of a LeFort I osteotomy is presented.