Within psychoanalysis acting out is, today, a contested concept, both in terms of its theoretical underpinnings and with regard to its clinical application. In light of this the present paper presents a review of the concept which begins with Freud and moves on to trace the various discussions and controversies which have surrounded the term. It is argued that acting out is a valid and clinically important psychoanalytic concept, though one which retains its value only in virtue of unpacking its relation to the transference. Furthermore it is contended that this relation was initially made clear by Freud, and that this notion has been successfully built on and elaborated by, in particular, Lacan. In the context of discussing acting out, the related concepts of acting in and enactment are examined. The former is seen as representing instances of expressive actualisation, while the latter is found to be wanting in conceptual clarity. Also discussed are the position of the analyst in relation to the transference, and more specifically the problems associated with countertransference based interventions, highlighted by Lacan.