Increasing emphasis has been placed in recent years on development of the theory of Environmental Impact Assessment (EIA), primarily as a consequence of increasing recognition that the theoretical basis of ‘state-of-the-art’ EIA is inadequately developed and detailed. This study reviews consideration given in the research literature to the role of science in EIA in order to identify implicit theories. It is suggested that there are two main interpretations of the role of science in EIA (EIA as applied science and EIA as civic science) and five distinct models are identified within these paradigms. These models appear to be based predominantly on existing philosophies of science (such as positivism or relativism) and simplistic and ill-defined conceptions of the purposes of EIA. A broad model is proposed for the advancement of theory regarding the role of science in EIA which emphasises conceptual consideration and empirical investigation of the purposes, and hence outcomes, of EIA and the causal processes utilised to achieve these purposes. The model necessitates a reorientation of the research agenda, away from process and procedure to focus on substantive purposes, and this will require more integrative and connective research than has been commonplace in the past. The EIA research agenda must evolve and mature if this globally significant decision tool is to fulfil its potential.