Prior to implementing interventions, practitioners must first develop definitions of behavior that are objective. This chapter first provides a definition of behavior and provides three criteria for determining whether something is a behavior: it is demonstrated by a living organism, in interaction with the environment, and the interaction results in measurable change within the environment. Next, the chapter outlines how practitioners should develop operational definitions of behaviors of interest. Then, the chapter provides a description of the dead man’s test, a heuristic that practitioners often utilize when determining if something qualifies as a behavior suitable for intervention. Finally, the chapter describes the meaningful operant dimensions of behavior, such as frequency, duration, latency, and magnitude.