The first migration of persons in independent Cambodia took place during the 1950s and 1960s, when ethnic Chinese were permitted to settle in the mountainous and wasteland areas and cultivate land that otherwise would have remained useless. After 1970, about 200,000 Vietnamese living in Cambodia were repatriated to the RVN, ostensibly as a security measure. With the insurgent victory in April 1975, most of the country's remaining Vietnamese were reported to have emigrated to Viet Nam. In addition, thousands of refugees, including many former officials and military personnel, fled across the Thai border or were evacuated by US aircraft.
The new government launched a sweeping nationwide resettlement program under which some 2.5–3 million persons were moved from Phnom Penh and other cities into the countryside, where they were organized into work brigades. The food shortage in rural areas was only slightly less critical than in the cities, and widespread starvation led to the deaths of probably over one million people during the transition. After the installation of the PRK in January 1979, continued fighting and political instability resulted in a new exodus of refugees. About 630,000 Cambodians left the country between 1979 and 1981, of which about 208,000 were able to resettle in other countries, including 136,000 in the United States. Most of the rest remained in camps on the border with Thailand, but they were repatriated to Cambodia in May 1993.
Between 1979 and 1987 there was a new migration of ethnic Vietnamese into Cambodia. Official sources insisted that the total number was under 60,000, and was comprised, for the most part, of residents who had left in the early 1970s, but opposition groups contended that the number totaled over 500,000 and was intended to consolidate Vietnamese control over the country.
In 1997, the conflicts between government forces and the National Army of Democratic Kampuchea (Khmer Rouge) drove rural populations from their homes. In 1997 and 1998, UNHCR assisted up to 60,000 Cambodian refugees who had the fled fighting in Northwest Cambodia. Also in 1997, the UNHCR helped several thousand ethnic Vietnamese fisher families return to their Cambodian homes after having camped on the Vietnam border. Following the peace settlement between the government of Cambodia and resistance forces in December 1998, the repatriation of approximately 36,000 refugees remaining in camps in Thailand was rapidly implemented. By April 1999, all of the camps were closed, and by June 1999 some 47,000 refugees had returned home. The net migration rate for Cambodia in 2000 was 0.7 per 1,000. In that year there were 211,000 non-citizen residents living in Cambodia. The government continues to view the emigration level as too high.