In United States constitutional law, police power is the capacity of the states to regulate behavior and enforce order within their territory for the betterment of the general welfare, morals, health, and safety of their inhabitants. Under the 10th Amendment to the United States Constitution, the powers prohibited from or not delegated to the Federal Government are reserved to the states respectively, or to the people. This implies that the states do not possess all possible powers, since some of these are reserved to the people.

The exercise of police power can be in the form of making laws, compelling obedience to those laws through legal sanctions, physical means, or other forms of coercion and inducements. Controversies over the exercise of police power, particularly the use of physical means, arise when its exercise by the federal government conflicts with the rights of the states or when its exercise by federal or state authorities conflicts with individual rights and freedoms.

Police powers are, from the point of view of state courts, also restricted by state constitutions. The concept of police power is used by federal courts which do not have jurisdiction to interpret state constitutions: from the point of view of federal constitutional law, states have general police powers except where restricted by the federal Constitution.

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.