Los Alamos (Spanish: Los Álamos, meaning "The cottonwoods") is a townsite and census-designated place (CDP) in Los Alamos County, New Mexico, United States, built upon four mesas of the Pajarito Plateau and the adjoining White Rock Canyon. The population of the CDP was 12,019 at the 2010 Census. The townsite or "the hill" is one part of town while White Rock is also part of the town. Technically, both are part of the same incorporated city/county. Los Alamos is home to the Los Alamos National Laboratory, which was founded to undertake the Manhattan Project. Los Alamos County is an incorporated county, and many county offices are located at the townsite of Los Alamos. Los Alamos High School is the public high school of Los Alamos County. Before the Manhattan Project, the site was occupied by the Los Alamos Ranch School. Los Alamos has a county council which the people elect to four-year terms. The county council is seven members with four or three persons elected every two years to the four-year terms. The council elects a chairman and vice-chairman.

Los Alamos is built on the Pajarito Plateau between White Rock Canyon and the Valles Caldera, part of the Jemez Mountains. The Jemez Mountains are predominantly formed by the 18.7 Ma to ~50 ka Jemez volcanic field. Volcanic activity in the Jemez Mountains culminated with the formation of two geographically coincident calderas, the 1.61 Ma Toledo caldera and 1.25 Ma Valles caldera, both of which lie to the west.[1]

The first settlers on the plateau are thought to be Keres speaking Indians around the 10th century. Around 1300, Tewa settlers immigrated from the Four Corners Region and built large cities but were driven out within 50 years by Navajo and Apache raids and by drought. Both the Keres and Tewa towns can be seen today in the ruins of Bandelier National Monument and Tsankawi.

The land of the plateau was then divided up for homesteading. Most residents of the plateau built simple log cabins that were only resided in during warm weather to feed livestock, with the homesteaders moving down to the warmer Rio Grande Valley. Homesteader Harold H. Brook sold part of his land and buildings to Detroit businessman Ashley Pond in 1917 which began the Los Alamos Ranch School, named after the cottonwood trees that blossomed in the fall. The school lasted for 25 years, when the Department of Defense was looking for a remote location for the Manhattan Project during World War II.

In 1942, the government used its power of eminent domain to take over the Ranch School and all the remaining homesteads. The Ranch School was paid $225 per acre while the homesteaders were paid only between $7 and $15 per acre. All information about the town was highly classified until the bombing of Hiroshima.

All incoming truckloads were labeled as common items to conceal the true nature of their contents, and any outbound correspondence by those working and living in Los Alamos was censored by military officials. At the time, it was referred to as "The Hill" by many in Santa Fe, and as "Site Y" by military personnel. The mailing address for all of Los Alamos was PO Box 1663, Santa Fe, NM. After the Manhattan Project completed, White Rock was abandoned until 1963 when people began to re-inhabit and rebuild new homes and buildings.

Los Alamos National Laboratory was established as a research government facility under the Department of Energy.

In 2000, a 48,000 acre wildfire, Cerro Grande Fire, tore through the townsite after the Park Service lit a prescribed fire in a high wind situation. The town was evacuated for 8 days while firefighters from all over the country battled the blaze, but over 400 homes were lost. FEMA built temporary housing for those that were displaced by the fire on North Mesa near the Middle School. It was nicknamed FEMA-ville. After the fires, there was widespread flooding due to the removal of the groundcover by the fire.

On Monday, June 27, 2011, the Las Conchas Fire broke out in the mountains southwest of the town, prompting a mandatory evacuation from the town and temporary closure of LANL until Sunday. Although Las Conchas was actually larger than that of the Cerro Grande Fire, no buildings or structures in Los Alamos County were damaged.