This paper examines timing constraints as features of realtime systems. It investigates the various constructs required in requirements languages to express timing constraints and considers how automatic test systems can validate systems that include timing constraints. Specifically, features needed in test languages to validate timing constraints are discussed. One of the distinguishing aspects of three tools developed at GTE Laboratories for real-time systems specification and testing is in their extensive ability to handle timing constraints. Thus, the paper highlights the timing constraint features of these tools.