This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book defines interest groups by explaining why people are so prominent in the United States. It explores how and why interest groups form, why they fail to form, and why they sometimes emerge out of social movements. The book describes how lobbyists interface with the members of interest groups, the people they are supposed to represent, and problems with representation quality as groups grow older and larger. It investigates how interest group lobbyists gain influence in the institutions of the national government, such as Congress, the executive branch and the judicial branch. The book shows that how employing various methods of influence can enhance or detract from the basic representative function of interest groups. It discusses how group lobbyists interact with each other through bargaining, conflict, and strategically choosing where to lobby.