Though use of theory is critical in Information Systems (IS) research, the theoretical foundations of IS research have been understudied. Using Social Network Analysis, we analyze theory usage in IS research published in MIS Quarterly and Information Systems Research from 1998 to 2006. We find Technology Acceptance Model, Resource- Based View and Game Theory to be the three most frequently used theories. While strong dominance is found in research focusing on Information Technology (IT) for individuals, organizations and markets, no theoretical dominance is found in IT for groups and IS development. Psychology, Economics and Sociology are disciplines IS researchers most frequently leverage for theories. Psychology contributes several theories representing a large fraction of the long tail of theories. Our analysis suggests that IS consists of a few distinctive clusters of research instead of a single core. Our results provide insights on theoretical foundations of IS and suggest research opportunities for scholars.