Turning passive into active is an ego function that plays an important role in managing anxiety associated with passive feelings of helplessness and powerlessness. Its use in childhood aids the ego in constructing a basic sense of mastery, a secure sense of being in control, both of oneself and of one's circumstances. Functioning first as a building block of ego, it subsequently becomes a versatile mechanism of defense with utility throughout the life cycle. This article discusses both the developmental and defensive uses of the passive into active mechanism of defense and illustrates them with clinical examples. Often regarded only as a byproduct of defense, its importance in mastering core developmental anxieties and their adult revivals warrants its inclusion in the glossary of discrete defenses.