There is an extensive literature on interlocking directorates that uses sophisticated methods of network analysis to map the structure of board interlocks among organizations and assess the implications of those interlocks for organizational behavior (Mizruchi, 1996). However, almost all of this literature -and, therefore, most of what we know about the nature and significance of interlocking directorates -is based on studies of a single type of organization: the large corporation. Like corporations, other kinds of organizations -colleges and universities, charities, civic organizations, foundations, think tanks, trade associations, political lobbies, and interest groups -also have governance structures in which professional administrators are overseen by boards of directors or boards of trustees that include members who are affiliated with outside organizations. As in the case of corporations, this governance structure creates the possibility of direct and indirect interlocks among organizations within a given domain. Unlike corporate board interlocks, however, these non-corporate networks, and their implications for organizational behavior, have received relatively little study. This paper seeks to advance our knowledge of one type of non-corporate interlocking directorate by presenting a network analysis of the governing boards of 12 leading policy-planning organizations and changes in the structure of this interlock network between 1973 and 2000. Privately financed and directed policy-planning organizations like the American Enterprise Institute, the Brookings Institution, and the