Interest groups are associations of individuals who have common goals and who work together to achieve their goals by attempting to influence government policy. These groups are also sometimes referred to as voluntary association or pressure groups. They are voluntary groups in that their members tend to join them out of their own free will because they see them as an effective way to achieve their political ends. They are pressure groups in that they tend to apply pressure on governing officials in their attempts to persuade them to seek out their desired political ends. In democratic societies, such groups are the primary mechanism for representing public opinion and/or articulating the demands of a particular group. Interest groups generally present the viewpoints of the citizenry they represent to the government in an organized, and thereby, effective way.

Although interest groups may articulate and aggregate the diverse interests found in society in the scope of the political arena, they are not synonymous with political parties. This is because they seek only to influence government in their member's favour, not to harness the power of government. Thus, once an interest group seeks control over the machinery of government, it is no longer an interest group, as it has, in effect, become a political party. However, this does not mean that interest groups will not work with political parties, or even become affiliated with one, to achieve their policy goals. Thus, the fundamental difference between interest groups and political parties is that interest groups are merely seeking to further their own interests in the political process and political parties are seeking to control the political process.

Interest groups are found in all states. However, their diversity and permitted activity tends to be quite limited in non-democratic or closed societies. To the extent that they exist in these societies, the significance of interest groups is minimal because these systems generally do not tolerate opposition in any form. Thus, the mandate of these groups is likely to be dictated by the government. Consequently, their autonomy to represent the interests of various groups in society will be substantially limited or non-existent in non-democratic systems.

Interest groups play a significant role in democratic political systems and policy processes. They provide the citizenry with an avenue of participation in the political system, and they are often the initiators of the political process through their articulation of demands to governing officials. Interest groups can get the government to look at issues of fundamental concern to their group members by persuading them to form public policy or legislation on the matter. Without interest groups bringing such issues to the forefront and forcing government officials to deal with them in a collective and organized method, the individual members of society who share the common goals of the group would likely not be able to get the government to deal with their concerns. Furthermore, the diverse and competing interests which are characteristic of a democratic society, necessitate a need for people to actively form associations to articulate their interests to government leaders effectively.

Interests groups employ a variety of tactics and engage in a variety of activities in their attempts to influence government policy to their own ends. For example, their activities may include undertaking direct political action, providing material resources such as goods and services to political actors, exchanging relevant data and information with those in the political system, and so on. Major groups with large enough numbers also have the valuable tool of cooperation. For instance, they may express their discontent with a particular policy by convincing the members of their group not to comply with the policy, thereby, making the policy unworkable in practice. Such action is intended to persuade the government to look into desired changes. These groups may also use their access to scarce, specialized information as a leverage tool on government. To some extent, they can withhold such information from government if the government refuses to address their problems or does not address them in what the group deems to be an acceptable manner.

Other ways these groups can affect the end result of the policy process are by electoral activities in which they may raise money, supply workers, or rally votes for a particular party that seems likely to favour their cause if elected. Another tactic is through the utilization of public information campaigns in which they attempt to influence policy indirectly by impacting upon the entire population. More dangerous tactics such as violence and the disruption of social order may also be used to dramatize the group's cause and show that it is willing to pay a high price for the attainment of its goals. Even litigation may be used by these groups to affect the development of policy by working within the court system. This may be important in setting a precedent in matters for which there is no pre-existing legislation or policy such as the recent Ontario court ruling that women can bare their breasts in public if they desire to do so. The tactics that interest groups can employ are endless. However, what does dictate which ones they will choose to employ is the source of its power whether that be from mass numbers, financial capabilities, or whatnot. Furthermore, the nature of the political system in which they exist will have a profound impact on what tactics they can use in the pursuit of their goals. After all, interest groups in authoritarian societies will be much more confined in how they can influence government policy than will be interest groups in democratic societies.

Despite all the other tactics available, lobbying is still the primary tool used by interest groups in democratic societies to get the government on side with the group in its policy decisions. The focus of this activity depends on the institutional arrangements of individual political systems, but the object of the activity is always to secure favourable policy decisions or the appointment of specific individuals to positions of power who are seen as supporters of the group's cause. This lobbying is done by representatives of the group through their dealings with government officials. This fundamental practice of interacting with government leaders to further the interests of the group is almost as old as contemporary interest groups themselves. In fact, its modern presence is derived from the old practice of individuals and groups seeking to influence government buttonholing Members of Parliament in the lobby of the British House of Commons. This entailed catching these MPs as they were going through the lobby, presenting them with the group's interest, and trying to influence them to support the group's cause.

Just as there are a variety of tactics at the disposal of interest groups, there are a variety of hindrances on their ability to represent public opinion. Naturally, these groups are not all equal in their structure or available resources. Thus, those with less organization, members, cohesion, wealth, leadership, and so on may not be able to have as strong a voice or command as much respect as other interest groups in the political system. Furthermore, even the nature of the issue of primary concern to the group is important in determining the degree of influence the group will have. After all, if the demands of the group are similar to what the government desires or is capable of providing, the group has a better chance of achieving its demands.

There are a variety of ways that one can classify interests groups. For example, there is the simple differentiation between public and private interest groups. The basic difference here is that the private groups seek only to advance the interests of their own group members and the public groups seek to promote causes that will bring benefits to all of society. An example of a private interest group would be a professional association that seeks to obtain such things as greater freedom from government regulation in the conduct of its profession or better wages for its professionals. Whereas, an example of a public interest group would be one that is concerned with broad social issues such as improving the state of the environment or the quality of consumer products. Since these groups are concerned with obtaining what is in the common good, they tend to lack the ability to bring immediate, tangible benefits to their members. There are also more complex systems of classifying these groups. For example, Professor Gabriel Almond classifies interest groups into four broad categories known as associational, non-associational, anomic, and institutional interest groups. This is the most commonly used classification system. However, no system is complete and within every category of possible classification there will always be differences among the groups that fall under its heading. For example, groups will differ in their reason for existence, in the focus of their activities, in their organizational assets, and so on.

Associational interest groups are often the political branch of a group that already exists for other reasons such as professional associations. Thus, they regard political activity as only one of their primary activities. These groups are characterized by their ongoing, formal organization which is a product of their efforts to influence public policy and articulate the interests of their members over the long term. This is the most common kind of interest group found in democratic societies and groups that fall under this category tend to have distinctive names, designated headquarters, and professional staff. For example, the NRA (National Rifle Association) in the United States.

Non-associational interest groups are in essence the complete opposite of associational interest groups. They lack any formal organization whatsoever, instead, they are composed of individuals who share some common, defining characteristic such as class, ethnicity, race, religion, culture, or gender. They seldom act as coherent political groups, but they are often treated by others as if they did. Despite their lack of political organization, the members of these groups tend to be seen as representatives of the group. These groups are of a latent nature in that although they may not currently be organized, that does not mean that they cannot become powerfully organized political forces under the right circumstances. Therefore, political leaders must take their special interests into consideration in the formation of public policy. These groups are present in every society and at times they may form temporary, loosely structured organizations to plan and coordinate political activity in an informal manner with regards to a particular issue. However, if this group becomes more formalized and enduring, it is transformed into an associational interest group.

Anomic interest groups are generally the result of turmoil and excitement. Consequently, their actions are often violent. They are characterized by their lack of formal organization, absence of obvious leaders, as well as their temporary and loose coordination of efforts. They are short lived, spontaneous aggregations of people who share a common concern over a particular issue. For instance, in the United States, the nationwide student demonstrations against the Vietnam War in the late 1960s and early 1970s stand as a primary example of this type of interest group. Despite their inherent lack of political organization, these groups can have an outstanding impact on political decisions. However, this tends to be the exception to the rule, as, for the most part, these groups are of little real importance. As a consequence, they tend to only attract minimal media attention and stand as mere indicators of public opinion.

Issue oriented groups have several characteristics in common with anomic groups but are far less volatile. For example, they share with anomic groups a lack of organization and cohesion, lack of endurance, a fluid membership, and a lack of governmental knowledge. As a consequence of their defining characteristics, they have major difficulties in forming and adhering to long term goals. Furthermore, they do not have a concern over disturbing their relationship with government as other interest groups do. The primary advantages of such groups are their great flexibility and tremendous ability to generate immediate public action on specific issues.

Institutionalized interest groups are characterized by well structured and enduring organization, stable membership, clear objectives, and exclusive knowledge of the appropriate sectors of government and their clients. They begin for purposes other than political activity and only engage in such activity in order to defend their own interests in the government's policy decisions. They are a part of government, departments or agencies, but they are politically neutral. For example, public service unions. Like other groups in society, they have particular concerns they want to see addressed and goals they want to pursue. However, as a part of government, unlike other groups, they tend to persuade government through internal means. Consequently, their activity is largely out of public view for the most part. There are some who take this definition of interests groups further to include such associational groups as aboriginal organizations which are closely associated with government through their receipt of government funding.

The phenomenon of interest groups seems to be exploding in democratic societies in the past few decades as a consequence of a variety of factors. For example, the rise in benefits provided for by the welfare state is leading interest groups to form to protect and extend those rights to their group members. Another reason is the decline of political parties which is leading interest groups to lobby government directly for their aspired goals. Furthermore, the growing complexity of society is giving way to a variety of single issue oriented groups. There is no one single factor for the increasing rise of interest groups. However, as they increasingly establish their role in the political system, there are some fundamental problems that need to be addressed. For instance, the leadership of these groups tends to lack democratic organization. Therefore, they may not actually present a true of picture of public opinion, but instead may demonstrate the desires of the leaders who articulate the group's policy interests to government. Furthermore, these groups vary in their possession of resources needed to give them influence. Consequently, the playing field of these competing groups is far from equal.