Los Angeles County (incorporated as the County of Los Angeles) is a county in California and is the most populous county in the United States. Figures from the U.S. Census Bureau give an estimated 2009 population of 9,848,011 residents, while the California Department of Finance lists a January 1, 2009, estimate of 10,393,185. The county seat is the city of Los Angeles, the largest city in California and the second-largest city in the United States.

The county is home to 88 incorporated cities and many unincorporated areas. The southern portion is the most heavily urbanized area and is home to the vast majority of the population which lives along the Southern California coastline and the inland basins and valleys. The northern half is a large expanse of less-populated desert including the Santa Clarita Valley and the Antelope Valley, which encompasses the northeastern part of the county and is adjacent to Kern County. In between these portions of the county sit the San Gabriel Mountains and the vast wilderness known as the Angeles National Forest.

The county is home to over a quarter of all California residents. One of the most diverse counties in the country, it holds most of the principal cities encompassing the Greater Los Angeles Area and is the core of the five counties that make up the area.