A matching-to-sample procedure that engenders observation of two elements of a complex sample stimulus may establish equivalence relations among those elements. Studies that focus on complex stimulus control will help determine the necessary and sufficient conditions for equivalence class formation. Further studies in this area will benefit by blending the methods and concepts of stimulus equivalence research with those used in studies of conceptually related learning processes. The enterprise may also profit by reconsidering the role of compound stimulus control in the formation of equivalence.