How people intentionally change addictive behaviors with and without treatment is not well understood by behavioral scientists. This article summarizes research on self-initiated and professionally facilitated change of addictive behaviors using the key trans-theoretical constructs of stages and processes of change. Modification of addictive behaviors involves progression through five stages--pre-contemplation, contemplation, preparation, action, and maintenance--and individuals typically recycle through these stages several times before termination of the addiction. Multiple studies provide strong support for these stages as well as for a finite and common set of change processes used to progress through the stages. Research to date supports a trans-theoretical model of change that systematically integrates the stages with processes of change from diverse theories of psychotherapy.