The U.S. Census Bureau lists 3,141 counties or county-equivalent administrative units total. The power of the county government varies widely from state to state, as does the relationship between counties and incorporated municipal governments. The distribution of power between the state government, county governments, and municipal governments is defined in each state's constitution.
Counties are usually governed by an elected board of supervisors, county commission, county council or county legislature. In some counties, there is a county mayor or a county executive. The position of mayor is mostly ceremonial in some states, while in others, the mayor is more powerful than the commissioners or supervisors.
In many states, the board in charge of a county holds powers that transcend all three traditional branches of government. It has the legislative power to enact ordinances for the county; it has the executive power to oversee the executive operations of county government; and it has quasi-judicial power with regard to certain limited matters (like hearing appeals from the planning commission if one exists).
As for the day-to-day operations of the county government, they are sometimes overseen by a county manager or chief administrative officer who reports to the board, the mayor, or both.
In some states, the county technically has a plural executive in that several important officials are elected separately from the board of commissioners or supervisors (implying they cannot be fired by the board). This can create tension if such officials then disagree on how to best carry out their respective functions.
In New England, counties function at most as judicial court districts and sheriff's departments (in Connecticut only as judicial court districts and in Rhode Island, they have lost both those functions) and most government power below the state level is in the hands of towns and cities. In several of Maine's sparsely populated counties, small towns rely on the county for law enforcement, and in New Hampshire several social programs are administered at the state level. In some New England states, such as Connecticut, parts of Massachusetts, and Rhode Island, counties are only geographic designations and do not have any governmental powers. All government is either done at the state level or at the municipal (town or city) level. In Connecticut, 15 regional councils have been established to partially fill the void left behind when the state abolished its eight county governments in 1960. Connecticut's regional councils do not conform to the old county lines, rather they are composed of towns that share the same geographic region and have similar demographics. The regional councils' authority is much more limited compared with a county government: the regional councils have no taxing authority or authority to issue permits; the aforementioned powers are delegated to the town governments. Regional councils do however have authority over infrastructure and land use planning, distribution of state and federal funds for infrastructure projects, emergency preparedness, and limited law enforcement.
Outside New England, counties typically provide, at a minimum, public utilities, libraries, hospitals, public health services, parks, roads, law enforcement, and jails. There is usually a county registrar, recorder, or clerk (the title varies) who collects vital statistics, holds elections (sometimes in coordination with a separate elections office or commission), and prepares or processes certificates of births, deaths, marriages, and dissolutions (divorce decrees). Other key county officials include the coroner/medical examiner, treasurer, assessor, auditor, controller, and district attorney.
In most states, the county sheriff is the principal law enforcement officer in the county.
In most western states, the county controls all unincorporated land within its boundaries. In states with a township tier, unincorporated land is controlled by the township. Residents of unincorporated land who are dissatisfied with county-level or township-level resource allocation decisions can incorporate as a city or village.
A few counties provide public transportation themselves; this usually comes in the form of a simple bus system. However, in most counties, public transportation is provided by one of the following: a special district that is coterminous with the county, a multi-county regional transit authority, or a state agency.
In some states, more populated counties provide many facilities, such as airports, convention centers, museums, beaches, harbors, zoos, clinics, law libraries, and public housing. They provide services such as child and family services, elder services, mental health services, welfare services, veterans assistance services, animal control, probation supervision, historic preservation, food safety regulation, and environmental health services. They have many additional officials like public defenders, arts commissioners, human rights commissioners, and planning commissioners. Finally, there may also be a county fire department (as distinguished from fire departments operated by individual cities, special districts, or the state government).
Maryland in particular vests its counties with broad powers, including police and educational responsibilities. Because of the large scope of services provided by Maryland's counties, there is little incentive for a populated place to incorporate and the vast majority of urban Marylanders other than those within Baltimore (which under Maryland law is itself a county-equivalent independent city) reside in unincorporated areas.
At the 2000 U.S. Census, the median land area of the 3,077 U.S. counties was 622 sq mi (1,611 km²), which is only two-thirds of the median land area of a ceremonial county of England, and only a little more than a quarter of the median land area of a French département. However, this figure does not account for the differences among the United States counties themselves; counties in the western United States have a much larger mean land area than those in the eastern United States. For example, the median land area of counties in Georgia is 343 sq mi (888 km²), whereas in Utah it is 2,427 sq mi (6,286 km²).
The largest county equivalent by (total) area is Yukon-Koyukuk Census Area, Alaska, at 147,843 square miles (382,912 km²), while the largest county is San Bernardino County, California, at 20,105 square miles (52,071 km²). The smallest county equivalent is the independent city of Falls Church, Virginia, at 2.0 square miles (5 km²), while the smallest county is Kalawao County, Hawaii, at 13 square miles (34 km²). The smallest self-governing county is New York County, New York, at 22.96 square miles (59.47 km²).
At the 2000 U.S. Census, only 16.7% of U.S. counties had more than 100,000 inhabitants. This reflects the essentially rural nature of U.S. counties, whose grid was designed in the 19th century, in a country still largely rural and only marginally affected by urbanization. Today, the vast majority of people in the United States are concentrated in a relatively small number of counties. The most populous county equivalent is Los Angeles County, California, with 10,226,506 inhabitants as of 2005, and the least populous county is Loving County, Texas, with 60 inhabitants as of 2005.
The most densely populated county (or county equivalent) is New York County, New York (coextensive with the Borough of Manhattan and consisting primarily of Manhattan island) with 66,940 people per square mile (25,846 per km², or 38.691 square meters per person) as of 2000, and the least densely populated county is Lake and Peninsula Borough, Alaska, with 0.0767 people per square mile (0.0296 per km², or 33.768 km² per person) as of 2000. The least densely populated county equivalent is Yukon-Koyukuk Census Area, Alaska, with 0.0449 per square mile (0.0173 per km², or 57.683 km² per person) as of 2000.