There is a growing interest in the use of theory in order to move the field of impact assessment forward, hereunder to advance our understanding of effectiveness and make impact assessment influence decision-making. However, the extent of use of theory is uncertain. This article examines the use of explicit theory from other fields in impact assessment literature and analyse how research uses theory along a continuum from use of non-attached theory to theory building. A sample of research articles (n = 177) published in the Journal of Environmental Assessment Policy and Management from 1999–2014 was reviewed to determine whether, how and which theories was used. Of the articles 32% incorporated theory, of which 28% only mention theories in the introduction or literature review, but without explicit evidence that the theory is used to any other purposes than to establish legitimacy or support the author's ideas of "non-attached theory" and 40% in which theories are used to inform and focus the design of the study. The results show no significant changes in the use of theory in the period 1999–2014. The most often used theories were political and learning theory. The article concludes that there is a broad scope for future use of theories from other fields to mature research and practice of impact assessment. There is no lack of relevant theories, so the underlying reasons for the found results shall be found elsewhere.