Advocacy groups, also known as lobby groups, interest groups, special interest groups, pressure groups, or public associations, use various forms of advocacy or lobbying to influence public opinion and ultimate public policy. They play an important role in the development of political and social systems.

In politics, lobbying, or advocacy, is the act of lawfully attempting to influence the actions, policies, or decisions of government officials, most often legislators or members of regulatory agencies, but also judges of the judiciary. Lobbying, which usually involves direct, face-to-face contact in cooperation with support staff that may not meet directly face-to-face, is done by many types of people, associations and organized groups, including individuals on a personal level in their capacity as voters, constituents, or private citizens; it is also practiced by corporations in the private sector serving their own business interests; by non-profits and non-governmental organizations in the voluntary sector through advocacy groups to fulfil their mission such as requesting humanitarian aid or grantmaking; and by fellow legislators or government officials influencing each other through legislative affairs in the public sector.