The test effort required to achieve full test coverage of an event-trigered real-time system is very high. In this dissertation, we investigate a method for reducing this effort by constraining behavior of the system. We focus on system level testing of applications with respect to timeliness. Our approach is to define a model for constraining real-time systems to improve testability. Using this model applicability of our constraints is easily determined because all the pertinent assumptions are clearly stated. We perform a validation of a test effort upper bound for event-triggered real-time systems with respect to this model. Effects that constraints for improving testability have on predictability, efficiency, and scheduling are investigated and validated. Specific design guidelines for selection of appropriate constraint values are presented in this work. Finally, we discuss mechanisms for handling constraint violations.