Recognition of the value of acceptance of the self, others, and the flux of human experience, has philosophical and religious roots that date back thousands of years. The past two decades have witnessed a swell of interest in acceptance, as evidenced by an increase in acceptance-based therapeutic interventions, and a growing appreciation of the paradoxical nature of acceptance and personal change and the role of experiential avoidance in psychopathology. In this article we review historical and contemporary descriptions and definitions of acceptance, measures of acceptance, the relation between acceptance and change, and the role of acceptance in psychopathology and psychotherapy. Our central objective is to delineate a rich conceptual scheme that encompasses the diverse ways in which acceptance has been explicated in classical and contemporary writings, and to highlight the need for further validation of this useful and popular construct.