Alameda County is a county in the state of California in the United States of America. As of the 2010 census, the population was 1,510,271, making it the 7th-most populous county in the state. Its county seat is Oakland.
The Spanish word alameda means "a place where poplar trees grow," a name originally given to the Arroyo de la Alameda (Poplar Grove Creek). The willow and sycamore trees along the banks of the river reminded the early explorers of a road lined with trees, also known as an alameda.
The county seat at the time it was formed was located at Alvarado; it was moved to San Leandro in 1856 where the county courthouse was destroyed by the devastating 1868 quake on the Hayward Fault. The county seat was then re-established in the town of Brooklyn from 1872-1875. Brooklyn is now part of Oakland, which has been the county seat since 1873.
Much of what is now considered an intensively urban region, with major cities, was developed as a trolley car suburb of San Francisco in the late 19th and early 20th centuries. The historical progression from native American tribal lands to Spanish, then Mexican ranches, then to farms, ranches, and orchards, then multiple city centers and suburbs, is shared with the adjacent and closely associated Contra Costa County.