The relapse of problem behavior after apparently successful treatment is an enduring problem for the field of applied behavior analysis. Several theoretical accounts of treatment relapse have emerged over the years. However, one account that has received considerable recent attention is based on behavioral momentum theory (BMT). BMT has shown that behavior is more persistent in contexts that are correlated with higher rates of reinforcers after disruption of the response–reinforcer relation. Accordingly, relapse after successful treatment can be viewed as the persistence of behavior when treatment is compromised in some manner. We review basic BMT research, alternative accounts of treatment relapse, and translational research studies derived from BMT research. The implications for applied behavior analysis in practice are discussed along with potential solutions to the problem of treatment relapse