How do advocacy groups become actively involved in national policymaking? Why are some of these non-governmental organizations able to become major players in Congress, the administration, and the courts while others remain peripheral participants in American politics? Current research, using surveys of organizations or case studies, emphasizes mobilization and strategy. Scholars seek to understand influence on policy outcomes but have yet to determine the factors that generate its precursor, active involvement in policymaking. I present an alternative theoretical and empirical approach. Adapting organizational and institutional theory, I argue that advocacy organizations succeed in Washington by becoming taken-for-granted position advocates in policy debates as representatives of public constituencies. An organization’s longevity, the scale of its Washington presence, the scope of its political agenda, and its formal ties to public supporters and policy expertise will govern its level of involvement in policymaking in all major venues. Using new data on the involvement of more than 1,600 advocacy organizations in Congressional testimony, presidential directives, administrative rulemaking, and federal litigation, I demonstrate that these hypotheses are largely correct. An organization’s age, the size of its political staff and issue agenda, and its ties to public membership and issue expertise are the primary determinants of its involvement in all branches of government, rather than its lobbyists or its Political Action Committee. Yet, due to barriers to participation and lack of policymaker control, the types of interests that are involved in agencies and courts are less representative of the organizational population than those involved in Congressional and Presidential policymaking.