This article seeks to delineate a general theory of interest-group dynamics in America since 1890. Interest groups are seen to act in issue areas which cycle through phases of business control to reform activity and back again. Economic producer groups have a more stable incentive to participate in issue-area decision making than the reform groups that challenge their control. However, after a few years of the business-control phase of the cycle, unchecked producer groups tend to commit ‘excesses’, violations of widely shared values. This leads to political participation by reformers, most of whom lose interest in issue-area participation after a few years. Across the scope of hundreds of issue areas, business control or reform phases tend to occur at the same time. This is an important cause of similar cycles in the politics of the entire system, as described by such writers as Samuel Huntington, Arthur Schlesinger, Jr and Albert Hirschman. Interest-group cycles theory is based on critical pluralism, a new outlook that incorporates concepts of collective-action paradoxes, business power and social movements into interest-group studies.