Law enforcement in the United States is one of three major components of the criminal justice system of the United States, along with courts and corrections. Although each component operates semi-independently, the three collectively form a chain leading from investigation of suspected criminal activity to administration of criminal punishment. Also, courts are vested with the power to make legal determinations regarding the conduct of the other two components.
Law enforcement operates primarily through governmental police agencies. 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. Other duties may include the service and enforcement of warrants, writs, and other orders of the courts.
Local government in the United States refers to governmental jurisdictions below the level of the state. Most states have at least two tiers of local government: counties and municipalities. In some states, counties are divided into townships. There are several different types of jurisdictions at the municipal level, including the city, town, borough, and village. The types and nature of these municipal entities varies from state to state. Many rural areas and even some suburban areas of many states have no municipal government below the county level. In other places consolidated city–county jurisdictions exist, in which city and county functions are managed by a single municipal government. In some New England states, towns are the primary unit of local government and counties have no governmental function but exist in a purely perfunctory capacity (e.g. for census data).
In the United States, a sheriff is a county official and is typically the top law enforcement officer of a county. Historically, the sheriff was also commander of the militia in that county. Distinctive to law enforcement in the United States, sheriffs are usually elected. The political election of a person to serve as a police leader is an almost uniquely American tradition. (The Honorary Police of Jersey, a British Crown Dependency in the Channel Islands, have been elected since at least the 16th century.)
The law enforcement agency headed by a sheriff is typically referred to as a sheriff's office. According to the National Sheriffs' Association (an American sheriff's advocacy group founded in 1940), there were 3,085 sheriff's offices and departments as of the end of 2008. These range in size from very small (one- or two-member) forces in sparsely populated rural areas to large, full-service law enforcement agencies, such as the Los Angeles County Sheriff's Department, which is the largest sheriff's office and the seventh largest law enforcement agency in the United States, with 16,400 members and 400 reserve deputies. The average sheriff's department in the United States employs 24.5 sworn officers.