Son Sen (Khmer: pronounced [son sen]; 12 June 1930 15 June 1997), alias Comrade Khieu () or "Brother Number 89", was a Cambodian Communist politician and soldier. A member of the Central Committee of the Communist Party of Kampuchea/Party of Democratic Kampuchea, the Khmer Rouge, from 1974 to 1992, Sen oversaw the Party's security apparatus, including the Santebal secret police and the notorious security prison S-21 at Tuol Sleng.

Son Sen was responsible for ordering the massacre of more than 100,000 in the Eastern Zone of Cambodia during the last six months of 1978.Sen was married to Yun Yat, who became the Party's minister of education and information. Along with the rest of his family, he was killed on the orders of Pol Pot during a 1997 factional split in the Khmer Rouge.

The Khmer Rouge (; French: [kmɛʁ ʁuʒ]; Khmer: ខ្មែរក្រហម, Khmêr Krâhâm [kʰmae krɑːhɑːm]; "Red Khmer") is the name that was popularly given to members of the Communist Party of Kampuchea (CPK) and by extension to the regime through which the CPK ruled Cambodia between 1975 and 1979. The name was coined in the 1960s by then Prime Minister Norodom Sihanouk to describe his country's heterogeneous, communist-led dissidents, with whom he allied after his 1970 overthrow.The Khmer Rouge army was slowly built up in the jungles of eastern Cambodia during the late 1960s, supported by the North Vietnamese army, the Viet Cong, the Pathet Lao, and the Chinese Communist Party (CCP). Although it originally fought against Sihanouk, on the advice of the CCP, the Khmer Rouge changed its position and supported Sihanouk after he was overthrown in a 1970 coup by Lon Nol who established the pro-American Khmer Republic. Despite a massive American bombing campaign (Operation Freedom Deal) against them, the Khmer Rouge won the Cambodian Civil War when they captured the Cambodian capital and overthrew the Khmer Republic in 1975. Following their victory, the Khmer Rouge, who were led by Pol Pot, Nuon Chea, Ieng Sary, Son Sen, and Khieu Samphan, immediately set about forcibly evacuating the country's major cities. In 1976, they renamed the country Democratic Kampuchea.

The Khmer Rouge regime was highly autocratic, totalitarian, xenophobic, paranoid, and repressive. Many deaths resulted from the regime's social engineering policies and the "Moha Lout Plaoh", an imitation of China's Great Leap Forward which had caused the Great Chinese Famine. The Khmer Rouge's attempts at agricultural reform through collectivisation similarly led to widespread famine, while its insistence on absolute self-sufficiency even in the supply of medicine led to the death of many thousands from treatable diseases such as malaria. The Khmer Rouge regime murdered hundreds of thousands of their perceived political opponents, and its racist emphasis on national purity resulted in the genocide of Cambodian minorities. Summary executions and torture were carried out by its cadres against perceived subversive elements, or during genocidal purges of its own ranks between 1975 and 1978. Ultimately, the Cambodian genocide led to the death of 1.5 to 2 million people, around 25% of Cambodia's population.

In the 1970s, the Khmer Rouge were largely supported and funded by the Chinese Communist Party, receiving approval from Mao Zedong; it is estimated that at least 90% of the foreign aid which was provided to the Khmer Rouge came from China. The regime was removed from power in 1979 when Vietnam invaded Cambodia and quickly destroyed most of the Khmer Rouge's forces. The Khmer Rouge then fled to Thailand, whose government saw them as a buffer force against the Communist Vietnamese. The Khmer Rouge continued to fight against the Vietnamese and the government of the new People's Republic of Kampuchea until the end of the war in 1989. The Cambodian governments-in-exile (including the Khmer Rouge) held onto Cambodia's United Nations seat (with considerable international support) until 1993, when the monarchy was restored and the name of the Cambodian state was changed to the Kingdom of Cambodia. A year later, thousands of Khmer Rouge guerrillas surrendered themselves in a government amnesty.

In 1996, a new political party called the Democratic National Union Movement was formed by Ieng Sary, who was granted amnesty for his role as the deputy leader of the Khmer Rouge. The organisation was largely dissolved by the mid-1990s and finally surrendered completely in 1999. In 2014, two Khmer Rouge leaders, Nuon Chea and Khieu Samphan, were jailed for life by a United Nations-backed court which found them guilty of crimes against humanity for their roles in the Khmer Rouge's genocidal campaign.