A special interest is a person, group, or organization attempting to influence legislators or other public officials in favor of one particular interest or issue. In the UK, a group which specifically aims to influence public policy is known as a pressure group. Examples of special interests might include a corporation lobbying to win a specific government contract; a trade association representing the interests of an entire industry seeking favorable tax policies or government regulations; groups representing various sectors of society, such as labor unions, senior citizens or persons with disabilities; or groups within the legislature or bureaucracy themselves.

Many scholars dislike the term "special interest", since it carries a loaded, negative connotation. (Among other things, it presumes that we know exactly what the "general interest" is.) Some return to an old term, "vested interests" or refer to "particularistic groups." In the academic literature, it has largely been replaced by the term "interest group". There is a lively debate amongst political scientists as to what exactly constitutes an interest group. Some hold that only groups with members (for instance, Common Cause or the National Rifle Association) are interest groups. Others feel that interest groups are any non-government groups that try to affect policy. Some people define it even more broadly, to include individual corporations, or even government agencies. Sometimes "interest groups" are used to refer to groups within society (e.g. seniors, the poor, etc.) who are not necessarily part of an organized group.

A study by Jon Agnone , a sociologist at the University of Washington, in 2004 compared the number of bills passed between 1960 and 1994 by the U.S. Congress with tactics used by green groups within the same year. The study showed that each protest raised the number of pro-environment bills passed by 2.2%, but that neither efforts at conventional lobbying on Capitol Hill nor the state of public opinion made any difference. The study concluded that direct action, like chaining oneself to a bulldozer or throwing paint over company executives, is more likely to influence environmental policy than talking to politicians. Agnone presented his results to the American Sociological Association on August 17, 2004 at their meeting in San Francisco.