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.

A police officer (also known as a policeman or policewoman, and constable in some forces, particularly in the United Kingdom and other Commonwealth nations) is a warranted employee of a police force. In the United States, "officer" is the formal name of the lowest police rank; in many other countries "officer" is a generic term not specifying a particular rank, and the lowest rank is often "constable". Police officers are generally charged with the apprehension of criminals and the prevention and detection of crime, and the maintenance of public order. Police officers may be sworn to an oath, and have the power to arrest people and detain them for a limited time, along with other duties and powers.

Some police officers may also be trained in special duties, such as counter-terrorism, surveillance, child protection, VIP protection, and investigation techniques into major crime, including fraud, rape, murder and drug trafficking.

Responsibilities of a police officer are varied, and may differ greatly from within one political context to another. Typical duties relate to keeping the peace, law enforcement, protection of people and property, and the investigation of crimes. Officers are expected to respond to a variety of situations that may arise while they are on duty. Rules and guidelines dictate how an officer should behave within the community, and in many contexts restrictions are placed on what the uniformed officer wears. In some countries, rules and procedures dictate that a police officer is obliged to intervene in a criminal incident, even if they are off-duty. Police officers in nearly all countries retain their lawful powers, while off duty.