Most current user interface specification languages and toolkits are based on serial, discrete, token exchange paradigms that perform an acceptable job of implementing traditional WIMP (Window, Icon, Menu, Pointer) interfaces. These tools, however, are ill suited to address the needs of emerging interaction styles such as virtual environments and interactive simulations. These interaction styles commonly rely upon: full duplex, asynchronous, interrelated dialogues; a blend of continuous and discrete inputs and responses; and, implicit commands and probabilistic input events. Some forms of non-WIMP interfaces, particularly interactive simulations, must also contend with real time processing constraints and deadline-based computations.This work proposes a specification paradigm, the SHADOW System, which directly addresses these issues. This system has been demonstrated to allow both the semantic meaning and behavior of all simulation elements to be clearly defined in a reusable fashion while providing support for good software engineering practices.