The largest of the ethnic groups in Cambodia are the Khmer, who comprise approximately 90% of the total population and primarily inhabit the lowland Mekong subregion and the central plains. The Khmer historically have lived near the lower Mekong River in a contiguous arc that runs from the southern Khorat Plateau where modern-day Thailand, Laos and Cambodia meet in the northeast, stretching southwest through the lands surrounding Tonle Sap lake to the Cardamom Mountains, then continues back southeast to the mouth of the Mekong River in southeastern Vietnam.
Ethnic groups in Cambodia other than the politically and socially dominant Khmer are classified as either "indigenous ethnic minorities" or "non-indigenous ethnic minorities". The indigenous ethnic minorities, more commonly collectively referred to as the Khmer Loeu ("upland Khmer"), constitute the majority in the remote mountainous provinces of Ratanakiri, Mondulkiri and Stung Treng and are present in substantial numbers in Kratie Province.
Approximately 17-21 separate ethnic groups, most of whom speak Austroasiatic languages related to Khmer, are included in the Khmer Loeu designation, including the Kuy and Tampuan people. These peoples are considered to be the aboriginal inhabitants of the land by the Cambodian authorities. Two of these highland groups, the Rade and the Jarai, are Chamic peoples who speak Austronesian languages descended from ancient Cham. These indigenous ethnic minorities haven't integrated into Khmer culture and follow their traditional animist beliefs.
The non-indigenous ethnic minorities include immigrants and their descendants who live among the Khmer and have adopted, at least nominally, Khmer culture and language. The three groups most often included are the Chinese Cambodians, Vietnamese and Cham peoples. The Chinese have immigrated to Cambodia from different regions of China throughout Cambodia's history, integrating into Cambodian society and today Chinese Cambodians or Cambodians of mixed Sino-Khmer ancestry dominate the business community, politics and the media. The Cham are descendants of refugees from the various wars of the historical kingdom of Champa. The Cham live amongst the Khmer in the central plains but in contrast to the Khmer who are Theravada Buddhists, the vast majority of Cham follow Islam.
There are also small numbers of other minority groups. Tai peoples in Cambodia include the Lao along the Mekong at the northeast border, Thai (urban and rural), and the culturally Burmese Kola, who have visibly influenced the culture of Pailin Province. Even smaller numbers of recent Hmong immigrants reside along the Lao border and various Burmese peoples have immigrated to the capital, Phnom Penh.
Ethnic Khmer
The Khmers are one of the oldest ethnic groups in the area, having filtered into Southeast Asia around the same time as the Mon. Most archaeologists and linguists, and other specialists like Sinologists and crop experts, believe they arrived no later than 2000 BCE (over four thousand years ago) bringing with them the practice of agriculture and in particular the cultivation of rice. They were the builders of the later Khmer Empire which dominated Southeast Asia for six centuries beginning in 802 CE, and now form the mainstream of political, cultural, and economic Cambodia.
The Khmers developed the first alphabet still in use in Southeast Asia which in turn gave birth to the later Thai and Lao scripts. The Khmers are considered by most archaeologists and ethnologists to be indigenous to the contiguous regions of Isan, southernmost Laos, Cambodia and Southern Vietnam. That is to say the Khmer have historically been a lowland people who lived close to one of the tributaries of the Mekong.
The Khmers see themselves as being one ethnicity connected through language, history and culture, but divided into three main subgroups based on national origin. The Khmer of Cambodia speak a dialect of the Khmer language. The Northern Khmer (Khmer Surin) are ethnic indigenous Khmers whose lands once belonged to the Khmer Empire but have since become part of Thailand. The Northern Khmer also speak the Isan language fluently.
Maintaining close relations with the Khmer of Cambodia, some now reside in Cambodia as a result of marriage. Similarly, the Khmer Krom are indigenous Khmers living in the regions of the former Khmer Empire that are now part of Vietnam. Fluent in both their particular dialect of Khmer and in Vietnamese, many have fled to Cambodia as a result of persecution and forced assimilation by Vietnam.