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A patrol is commonly a group of personnel, such as law enforcement officers, military personnel, or security personnel, that are assigned to monitor or secure a specific geographic area.
From French patrouiller, from Old French patouiller “to paddle, paw about, patrol”, from patte “a paw”.
In military tactics, a patrol is a sub-subunit or small tactical formation, sent out from a military organization by land, sea or air for the purpose of combat, reconnaissance, or a combination of both. The basic task of a patrol is to follow a known route with the purpose of investigating some feature of interest or, in the assignment of a fighting patrol (U.S. combat patrol), to find and engage the enemy. A patrol can also mean a small cavalry or armoured unit, subordinate to a troop or platoon, usually comprising a section or squad of mounted troopers, or two armoured fighting vehicles (often tanks).
In non-military law enforcement, patrol officers are police officers (or, for private entities, security guards) assigned to monitor specified geographic areas. In this instance, patrol refers to the action of patrolling—that is, to move through their assigned areas at regular intervals to detect or prevent violations of the law or problems of any kind.
Patrol officers are the most recognizable members of the police, and are the government officials encountered most frequently by the public. Their duties include responding to calls for service, making arrests, resolving disputes, issuing tickets, taking crime reports, conducting traffic enforcement, investigating crimes, and conducting crime prevention measures. A patrol officer is often the first responder on the scene of any incident, and their actions can greatly affect the outcome of the investigation, as well as the lives and safety of themselves and others. Patrols are often done with the intent of community policing, to improve relations between police and the public. 
Patrol officers may conduct patrols on foot, while mounted, riding a police motorcycle or bicycle, driving a police car, crewing a police watercraft, or piloting a police aircraft, depending on the unit they are assigned to or their agency's capabilities. They may or may not be armed or uniformed. A study conducted by Temple University and the Philadelphia Police Department in the mid-2000s has shown that foot patrols reduce crime more than other methods. 
Some elementary schools utilize the term patrol to refer to students who are selected to monitor safety in the classroom or to those students who assist crossing guards with safety of children crossing busy nearby streets. Another common term for this use of patrol is hall monitor .
In Scouting, a patrol is six to eight Scouts (youth members) under the leadership of one of their number who is appointed Patrol Leader and supported by a Second or Assistant Patrol Leader. This is the basic unit of a Scout troop. The patrol method is an essential characteristic of Scouting by which it differs from all other organizations, using the natural dynamics of the gang for an educational purpose. 
Military police (MP) are law enforcement agencies connected with, or part of, the military of a state. In wartime operations, the military police may support the main fighting force with force protection, convoy security, screening, rear military reconnaissance, logistic traffic management, counterinsurgency, and detainee handling.
The United States Constabulary was a United States Army military gendarmerie force. From 1946 to 1952, in the aftermath of World War II, it acted as an occupation and security force in the U.S. Occupation Zone of West Germany and Austria.
The Massachusetts State Police (MSP) is an agency of the Commonwealth of Massachusetts' Executive Office of Public Safety and Security, responsible for law enforcement and vehicle regulation across the state. As of 2022, it has 2,067 troopers, 1,500 of them being uniformed troopers, and 611 civilian support staff, making it the largest law enforcement agency in New England. The MSP is headed by Interim Lieutenant Colonel Jack Mawn.
A border guard of a country is a national security agency that performs border security. Some of the national border guard agencies also perform coast guard and rescue service duties.
The United States Army Military Police Corps (USAMPC) is the uniformed law enforcement branch of the United States Army. Investigations are conducted by Military Police Investigators under the Provost Marshal General's Office or Special Agents of the United States Army Criminal Investigation Division (USACID).
The Florida Highway Patrol (FHP) is a division of the Florida Department of Highway Safety and Motor Vehicles. It is Florida's highway patrol and is the primary law enforcement agency charged with investigating traffic crashes and criminal laws on the state's highways.
The South Carolina Highway Patrol is the highway patrol agency for South Carolina, which has jurisdiction anywhere in the state except for federal or military installations. The Highway Patrol was created in 1930 and is an organization with a rank structure similar to the armed forces. The mission of the South Carolina Highway Patrol includes enforcing the rules and regulations in order to ensure road way safety and reducing crime as outlined by South Carolina law. The Highway Patrol is the largest division of the South Carolina Department of Public Safety and its headquarters is located in Blythewood. This department also includes the South Carolina State Transport Police Division, and the South Carolina Bureau of Protective Services.
The Oklahoma Highway Patrol (OHP) is a major state law enforcement agency of the government of Oklahoma. A division of the Oklahoma Department of Public Safety, the OHP has traffic enforcement jurisdiction throughout the state. OHP was legislatively created on July 1, 1937, due to the growing problem of motor vehicle collisions, the expansion of highway systems, and the increase in criminal activities.
The Maine State Police (MSP) is the state police agency for Maine, which has jurisdiction across the state. It was created in 1921 to protect the lives, property, and constitutional rights of the citizens of the state of Maine.
The Mississippi Highway Patrol is the highway patrol and acting state police agency for the U.S. state of Mississippi, and has law enforcement jurisdiction over the majority of the state.
The North Carolina State Highway Patrol (NCSHP) is the highway patrol agency for North Carolina which has no per-se "state police" agency. The Patrol has jurisdiction anywhere in the state except for federal or military installations and on the Cherokee Indian Reservation or on Cherokee outlying lands in the western mountains. The Highway Patrol was created in 1929 and is a paramilitary organization with a rank structure similar to the armed forces. NCSHP personnel at times conduct formations, inspections, honor guard activities and drill similar to the armed forces drill and ceremonies. Troopers have a reputation in North Carolina for immaculate uniform and grooming standards. The primary mission of the North Carolina State Highway Patrol is to ensure safe and efficient transportation on the streets or highways, reduce crime, protect against terrorism, and respond to natural and man-made disasters.
Law enforcement in Pakistan is one of the three main components of the criminal justice system of Pakistan, alongside the judiciary and the prisons. The country has a mix of federal, provincial and territorial police forces with both general and specialised functions, but the senior ranks of all the provincial forces and most of the federal ones are manned by members of the Police Service of Pakistan (PSP). The PSP is one of the most prestigious parts of the Central Superior Services, Pakistan's main civil service organisation. Federal law enforcement agencies are generally overseen by the Ministry of Interior of the Government of Pakistan, while provincial police forces are overseen by a department of the government of that province.
Provosts are military police (MP) whose duties are policing solely within the armed forces of a country, as opposed to gendarmerie duties in the civilian population. However, many countries use their gendarmerie for provost duties.
Police departments in the University of California system are charged with providing law enforcement to each of the system's campuses.
The Chicago Housing Authority Police Department (also known as the CHAPD) was created as a supplement to the Chicago Police Department (CPD), to provide dedicated police services to the residents of one of the nation's most impoverished and crime ridden developments for low-income housing. The CHAPD accomplished their daily goals by utilizing "community oriented policing techniques and aggressive vertical patrol" of all Chicago Housing Authority public housing projects throughout the inner city of Chicago, Illinois and some suburban areas.
Airport police units are a security police agency assigned to perform law enforcement functions at airports. They provide a wide range of law enforcement duties and responsibilities including patrol, investigation, traffic flow management, and control and response to airport emergencies. Airport police provide enhanced safety to airport employees, and to passengers. Officers can be found at security gates, throughout the terminal area, and around the airport’s perimeter.
The Pentagon Police Division (PPD) is the uniformed division of the Pentagon Force Protection Agency (PFPA).
National Police Corps, colloquially in English as Dutch National Police or National Police Force, is divided in ten regional units, a central unit, the police academy, police services center, and national control room cooperation. The law-enforcement purposes of these agencies are the investigation of suspected criminal activity, referral of the results of investigations to the courts, and the temporary detention of suspected criminals pending judicial action. Law enforcement agencies, to varying degrees at different levels of government and in different agencies, are also commonly charged with the responsibilities of deterring criminal activity and preventing the successful commission of crimes in progress. The police commissioner in the Netherlands is Henk van Essen since May 1, 2020.
The New York City Police Department (NYPD) is structured into numerous bureaus and units. As a whole, the NYPD is headed by the Police Commissioner, a civilian administrator appointed by the Mayor, with the senior sworn uniformed officer of the service titled "Chief of Department". The Police Commissioner appoints the First Deputy Commissioner as the department's second-in-command and the Chief of Department as the department's highest ranking uniformed officer. The commissioner also appoints a number of deputy and assistant commissioners who do not have operational command and are solely for support and administrative function. The department is divided into twenty bureaus, six of which are enforcement bureaus. Each enforcement bureau is further subdivided into sections, divisions, and units, and into patrol boroughs, precincts, and detective squads. Each bureau is commanded by a bureau chief. There are also a number of specialized units that are not part of any of the bureaus and report to the Chief of the Department.
In the United States, the state police is a police body unique to each U.S. state, having statewide authority to conduct law enforcement activities and criminal investigations. In general, state police officers or highway patrol officers, known as state troopers, perform functions that do not fall within the jurisdiction of the county sheriff, such as enforcing traffic laws on state highways and interstate expressways, overseeing the security of the state capitol complex, protecting the governor, training new officers for local police forces too small to operate an academy and providing technological and scientific services. They support local police and help to coordinate multi-jurisdictional task force activity in serious or complicated cases in those states that grant full police powers statewide.