This monograph focuses on a proposal for a testing model in safety critical systems. Due to the large scope of these systems, we have focused on the system testing and we have included requirements for testing the communication subsystem. After establishing the theoretical background for testing, we have defined the differences and specifics of traditional software systems and safety critical systems, based on standards and guidelines analysis for various safety critical systems. The system testing that we are using is not clearly integrated into the process of design and development of safety critical systems. Therefore, we have proposed basic steps of this process to which we have integrated system testing. Given the scope of the system testing, we have decided to propose two testing models. We have focused on performance and step stress testing. These models implement requirements for testing of safety critical systems specified by us. To verify the proposed models, we have defined a metric. Based on its value we can determine whenever the proposed model meets testing requirements specified by us.