County police
County police, often called county sheriffs in the United States, are police forces existing primarily in the United States that possess primary jurisdiction over an entire county. England and Wales, two constituent countries of the United Kingdom, are policed by territorial police forces which are largely formed on a county basis. Historically, Northern Ireland and Scotland, the other two constituent countries of the UK, have had county police, although both countries now have unified national police services. Sweden 
Until the end of 2014, all of the 21 counties of Sweden had its own County Police Department. In 2015, Sweden merged all local police departments into a single police agency, dividing the country into seven police regions instead. United Kingdom

 In England, the police are divided into 39 regional forces, which all provide full services throughout their districts. In Wales, the police are divided into 4 regional forces, which all provide full services throughout their districts. In Northern Ireland and Scotland, the police are national forces, which provide full services to the full country. Territorial police forces
Territorial police forces that have a presence at a regional and county level in England and Wales are:

 Avon and Somerset Constabulary
 Bedfordshire Police
 Cambridgeshire Constabulary
 Cheshire Constabulary
 City of London Police
 Cleveland Police
 Cumbria Constabulary
 Derbyshire Constabulary
 Devon and Cornwall Police
 Dorset Police
 Durham Constabulary
 Dyfed–Powys Police 
 Essex Police 
 Gloucestershire Constabulary 
 Greater Manchester Police 
 Gwent Police 
 Hampshire Constabulary 
 Hertfordshire Constabulary 
 Humberside Police 
 Kent Police 
 Lancashire Constabulary 
 Leicestershire Police
 Lincolnshire Police 
 Merseyside Police 
 Metropolitan Police 
 Norfolk Constabulary 
 North Wales Police 
 Northamptonshire Police 
 Northumbria Police 
 North Yorkshire Police 
 Nottinghamshire Police 
 Police Service of Northern Ireland 
 Police Scotland 
 South Wales Police 
 South Yorkshire Police
 Staffordshire Police 
 Suffolk Constabulary 
 Surrey Police 
 Sussex Police 
 Thames Valley Police 
 Warwickshire Police 
 West Mercia Police 
 West Midlands Police 
 West Yorkshire Police 
 Wiltshire Police

United States

In the United States, the powers, duties, and even existence of county police forces vary widely depending on the state, and even on the particular county (parish in Louisiana) within a state. County police forces as autonomous entities are relatively rare and tend to exist only in metropolitan counties. Many states also have a county sheriff's office, which is usually formed on a county basis and traditionally carries out duties related to the functioning of the courts and legal system, such as service of process, executing legal writs, and protection for the local courthouse and its judges. Many state constitutions mandate that the position of sheriff be created, which leads many states to also give sheriffs the duties of a county police to avoid having overlapping departments with similar duties, leaving the sheriff as the exclusive law enforcement agency for a county. Some counties that previously had both a county police force and a sheriff's office have merged the two, leaving the sheriff in command of the unified force, as the sheriff derives his power directly from the constitution; the most prominent example of such a merger is the Las Vegas Metropolitan Police Department which is actually led by the county sheriff despite its name. County police tend to fall into three broad categories:
Full-service police departments, which provide the full spectrum of police services to the entire county, irrespective of local communities, and may provide contractual security police services to special districts within the county. Hawaii has only county police; there are no local police. Limited service police departments, which provide services to unincorporated areas of the county (and may provide services to some incorporated areas by contract), and usually provide contractual security police services to special districts within the county. Restricted service police departments, which provide security police duties to county owned and operated facilities and parks. Some may also perform some road patrol duties on county built and maintained roads, and provide support to municipal police departments in the county. Note: County detectives, who are maintained in the northeastern states by county attorneys' offices, fall within this category. In the state of Louisiana, a county is known as a parish. In the state of Alaska, a county is known as a borough. The only borough in Alaska to have their own police department is North Slope Borough

References

Law enforcement units