Over the last decade numerous research and industry reports have highlighted the importance of cooperation to construction project success. Cooperation is routinely cited for good performance and its absence blamed for poor performance of construction projects. Yet most of these studies have not directly measured cooperation and where this has been attempted, the measures used have lacked a unifying theoretical framework. Instead, researchers have relied on project performance measures (e.g. time, cost and quality) as indicators of the level of cooperation, if any, which existed. Important as they are, these project performance measures provide information about the past and any lessons learned can only influence the next project. These measures also confound the effects of a number of other input factors (e.g. procurement methods, project contextual conditions). As such, they do not provide realtime and accurate information to aid project managers in making proactive decisions to influence the outcomes of their projects. A definition of, a theoretical framework for, and the defining aspects of cooperation are set out in this paper. Since cooperation at any level must ultimately be reduced to cooperation between individuals (e.g. managers from different organisations), a micro-level perspective is required in a thorough study of the antecedents of cooperation. Such understanding is necessary to improve both the predictability and quality of construction project performance.