For safety critical systems the granting of a certain dependability is necessary and represents the basic condition for the maintenance of a safety critical system operation. The next point is to handle the underlying functionality. In order to prevent faults or to mask those, it is essential to take measures and precautions in order to ensure high system dependability. The complexity of safety critical systems can only be handled by using a suitable methodology during the design process being supported by an adequate tool environment. The tool supports the design of safety critical systems.