Among the oil and gas exploration community, the "play" has an almost mythical status-the successful play is the thing of which legends are made, and playmakers are regarded as heroes of the industry. But what is the play exactly and why do we need it? Curiously, considering the long period it has been in daily use, it has never really been unambiguously defined and, as a result, it has come to be used differently for different purposes. Although the term is in common use, in practice, its imprecision commonly leads us to simply ignore its significance as a concept in our rush to concentrate on prospect definition. These questions arise: Do we really need the play, and can we gain by defining it more precisely? I strongly believe that the play concept is such a valuable one that it should be central to exploration decision making; clustering petroleum accumulations into natural families helps us manage the risks inherent in new and existing venture evaluation. However, I believe that plays can best help us in this way if they comprise meaningful natural groups that we can use both for reliable analog comparison and in meaningful statistical analysis. In this review, I propose a three-tier hierarchic framework for play definition based on (1) the petroleum charge system, (2) the reservoirseal formation pair or lithofacies, and (3) the trap type. These tiers can be related to the geodynamic, sedimentary, and tectonic events that drive phases in basin evolution, thus placing the concept directly in its geologic context. Copyright ©2010. The American Association of Petroleum Geologists. All rights reserved.