This paper describes a new system to automatically generate, evaluate and execute assembly sequences. With the commands generated by the system, robots are enabled to assemble complex products without explicit robot programming. Our approach uses CAD-models, symbolic spatial relations and a robot work cell description as input. The system provides a user-friendly interface to define the goal state of the parts to be assembled. The automatically generated and decomposed assembly plans can be executed by robots by means of a set of predefined skill primitives. For this purpose we classify the robot tasks by analyzing the symbolic spatial relations between the objects to be assembled, the depart-spaces and the necessary tools. The tasks are decomposed into suitable elementary robot operations, the skills, automatically. For guiding the robot during assembly we employ internal and external sensors.