||Complex reactive systems require specifying the relationship of inputs and outputs in the time domain. Typically, such descriptions involve complex sequences of events, often with explicit timing constraints, that combine to form the system's overall behavior. In addition, most reactive systems are highly concurrent and hierarchical. Investigating formal methods for behavior specification of reactive systems presents real challenges. A new visual language for behavioral specification of complex reactive systems is described. We call it Constraint Narrowing Grammar (CNG). CNG combines concepts from logic programming, rewriting, and lazy evaluation. The non-deterministic and non-terminating properties of CNG make it a nice formalism to model infinite reactive systems' behavior. We demonstrate how a graphical specification can be mapped to CNG rules and show step-by-step how notations of hierarchy, concurrency, and event communication are introduced into the specification. The resulting grammar is an executable behavioral specification of complex reactive systems.