Police, agents of a government or other authority who are in charge of enforcing laws and keeping order. Examples of public police forces include municipal, county, state, and federal police. Private police forces, called special function police—such as railway police, campus police, and harbor police—provide police services to nongovernmental institutions and have the same legal authority as regular police.

In democratic countries the main purpose of police is to protect the public against behavior or conditions that the public considers dangerous or disruptive. The police are traditionally concerned with halting crime—behavior that is unlawful. They prevent and detect crimes, arrest criminals, hold prisoners, and recover stolen property. They may also perform such public services as controlling traffic, helping the injured, and finding missing persons, and they may become involved with educational, social assistance, and crime prevention programs.

In some countries, secret police are used to help those in power exploit and control the public, largely by helping prevent social, economic, religious, or political change. A police state is one in which such police are used to control almost every aspect of people's lives.

In many countries, such as France, police are centralized; they are responsible to the national government rather than the local community. In the United States, Canada, and other countries with British political traditions, police are largely decentralized. Britain's police are under local control except for special forces such as the London Metropolitan Police and the City of London Police. The national government inspects local forces to see that they meet national standards, and it pays about half their expenses.

This article describes the organization and operations of local, state, and federal police in the United States.