Advocacy groups (also known as pressure groups, lobby groups, campaign groups, interest groups, or special interest groups) use various forms of advocacy in order to influence public opinion and/or policy. They have played and continue to play an important part in the development of political and social systems. Groups vary considerably in size, influence, and motive; some have wide ranging long term social purposes, while others are focused on and are a response to an immediate issue or concern.

The term interest group refers to virtually any voluntary association that seeks to publicly promote and create advantages for its cause. It applies to a vast array of diverse organizations. This includes corporations, charitable organizations, civil rights groups, neighborhood associations, professional and trade associations.