An interest group (also called an advocacy group, lobbying group, pressure group (UK), or special interest) is a group, however loosely or tightly organized, doing advocacy: those determined to encourage or prevent changes in public policy without trying to be elected.

Interest groups are set up to represent a particular group in society with a common interest. Interest groups attempt to influence Congress members’ decisions regarding public policy. The main function of interest groups is to provide information. They provide information to Congress members, executive agencies, and to constituents. There are three types of information acquired by interest groups. The first type of information that interest groups are concerned with acquiring is about the status and prospect of bills under active consideration. Interest groups also research the electoral implications of legislators’ support or opposition to that bill. Lastly, they analyze the political, economic, social, and environmental consequences of those policies.

Since members of Congress and constituents often do not know anything about a certain issue, interest groups have the advantage and are free to give them only the information they want them to have. This creates informational asymmetry and gives interest groups the advantage over other actors. Interest groups generally provide biased information, but members of Congress listen to interest groups because they have no one else to inform them of the issue. Also, if a member of Congress listens to an interest group, the interest group will provide information to their constituency that will help get them re-elected. Members of Congress have a staff that does reseach to provide them with information on the subject.

There are three general guidelines that must be met in order for a group to be considered an interest group. Interest groups must not appear on the election ballot or run for political office, their primary goal must be to attempt to influence legislative, executive, or judicial decisions, and they must be external to the institution of government. In addition to these guidelines, the two fundamental goals of interest groups are to gain access and influence. Access can be measured by the amount of time a member of Congress spends with members from an interest group.

Interest groups can exert influence in the form of amicus curiae (friend of the court) briefs. By doing this, interest groups act as direct representatives or sponsors of litigation. Another way interest groups can participate in the court is by sending sponsors of litigation. Here, they act as a legal service to a certain party in a case. For instance, if a person believes their constitutional rights were violated and are suing in a court, then an interest group, (in this case the ACLU would be probably provide the legal representation) could argue on their behalf in front of the court, or provide the financial resources for the individual to obtain the legal resources. However, the most important thing an interest group can provide to a party in the court room is legal and policy expertise. The most common way to do this is for an interest group's legal staff to prepare a brief and lend advice to another group who will then argue on behalf of that interest group. As a result of the extreme legal expertise needed in these instances, the interest groups participating in writing briefs must have substantial knowledge of the social or economic point that they are arguing.[1]

An example of how interest groups can effectively influence Congress is the Family and Medical Leave Act (FMLA). The successful passage of this bill was largely due to the organized efforts of interest groups. This example illustrates important points about interest groups. First, interest groups choose their areas of participation carefully based on their resources. Secondly, interest groups acquire a great deal of experience and expertise on issues before appearing before committee hearings.

Milbraith’s minimal effects hypothesis contends that interest groups don’t actually change the minds of Congress members because interest groups only appeal to members who already agree with them therefore they are not influencing anything. However, it can be argued that interest groups, although not changing the minds of members, do change the composition of Congress. Interest groups are more likely to give money to members who agree with them, which gives that member an increased likelihood of being elected. Interest groups are also much more likely to only give money to incumbents because they are already in a position to get things done, and giving money to someone who is not in office is too much of a gamble.