It is widely maintained that the study of policy alternatives, particularly if they are associated with introducing new tehcnologies that may engender vast social and environmental repercussions, ought to be interdisciplinary. There is, however, much confusion in the literature as to what exactly is meant by the term interdisciplinary. In the present paper, we quantitatively assess the extent of interdisciplinarity of studies and of research programs. First, we propose several working definitions of the concept of interdisciplinarity. Second, we consider the construction of indicators that quantify these definitions. Third, as an example, we examine whether or not a given policy oriented research program is truly interdisciplinary.