Disequilibrium theory is an approach to reinforcement that reconsiders the putative response strengthening prowess of stimuli. This disequilibrium approach-the pinnacle of the response deprivation hypothesis-reliably predicts changes in behavior without reference to a response strengthening process. While the strengthening model of reinforcement has received renewed and critical appraisal in behavior analysis, its appraisers have not fully considered the role that a disequilibrium conceptualization might play in their respective theories of reinforcement. In this essay we celebrate William Timberlake's legacy by elucidating the assumptions of disequilibrium theory and by exploring its predictions and implications within behavior analysis. We treat the disequilibrium approach to reinforcement as the theory of reinforcement in behavior analysis, and in doing so, we distinguish disequilibrium conditions from motivating operations and explore future directions regarding the potential to predict generalization and maintenance outcomes. The disequilibrium approach to reinforcement is not a mere deprivation operation used for the purposes of establishing a stimulus as a "reinforcer," as it is a general theory of behavior.