The police are a constituted body of persons empowered by the state to enforce the law, protect property, and limit civil disorder. Their powers include the legitimized use of force. The term is most commonly associated with police services of a state that are authorized to exercise the police power of that state within a defined legal or territorial area of responsibility. Police forces are often defined as being separate from military or other organizations involved in the defense of the state against foreign aggressors; however, gendarmerie and military police are military units charged with civil policing.

Law enforcement, however, constitutes only part of policing activity. Policing has included an array of activities in different situations, but the predominant ones are concerned with the preservation of order. In some societies, in the late 18th and early 19th centuries, these developed within the context of maintaining the class system and the protection of private property. Some parts of the world may suffer from police corruption.

Metropolitan Police is a generic title for the municipal police force for a major metropolitan area, and it may be part of the official title of the force. They provide all general police services and tend to be headed by a chief called a Commissioner or Sheriff.

In the United States, metropolitan police agencies have often been formed through mergers between several local police agencies formerly holding jurisdiction within neighboring communities within a metropolitan area or county, such as several local police departments and possibly the county sheriff's office.

Typically, such communities have experienced recent population growth and urban sprawl, which causes the area to more closely resemble and function as one single conurbation. Under these circumstances, a single law enforcement agency with a unified command, jurisdiction and support structure comes to be seen as more appropriate and efficient.

A related concept exists in some American counties in which county sheriffs' offices contract with some (though not necessarily all) local cities or towns within their counties to provide all law enforcement services in those municipalities in lieu of a separate city or town police force.