This article first outlines the major indirect sources and pathways of influence that interest groups use outside and within the administrative process to influence agency rulemaking. After reviewing the growth and changing nature of interest groups following and involved in rulemaking in Washington, it presents brief synopses of the avenues of influence available to them—presidency, the courts, and the Congress. A discussion of the more direct avenues for interest group influence is also presented, affording an overview of the rulemaking process. The four major opportunities for interest group participation and influence in the rulemaking process are reported: the agenda-setting stage, the pre-proposal stage, the notice-and-comment stage, and regulatory negotiation. It further offers an explanation of what is seen as the most profitable principles to apply in future research on this vastly understudied yet vital topic in American bureaucracy.