Phnom Penh is the largest city along the Mekong River. Its population fluctuated wildly during the 1970s and 1980s; the population of 1.2 million in 1971 swelled with war refugees to 2 million or more by 1975, when it was evacuated to almost nothing by the victorious Khmer Rouge communists. From 1978 (the last year of the Khmer Rouge regime) to 1987, Phnom Penh's population is estimated to have grown from about 50,000 to 700,000 people. The 2022 population was over 2.4 million.

A note on terms: Phnom Penh is pronounced p-NOM PEN. Phnom means "hill" or "mountain" in Khmer; Penh is a woman's name. More than 90% of Cambodians are ethnic Khmer, and Khmer is the national language. Cambodia has also been known as Kampuchea.