Population is unevenly distributed, with the highest density in lowland areas. Most of the people live in small agrarian villages; the largest cities are Jakarta, Indonesia; Bangkok, Thailand; Singapore; Manila, Philippines; and Ho Chi Minh City, Vietnam. There is a great diversity in culture, history, religion, and ethnic composition. Many different languages are spoken, such as those of the Tibeto-Burman, Mon-Khmer, and Malayo-Polynesian families. Religions include Buddhism, Hinduism, Islam, Roman Catholicism, and Confucianism. Animism is still practiced among many more isolated peoples of the region.

Most of the influences that molded the societies of Southeast Asia predate European colonization, coming from early Chinese and Indian sources. Several great civilizations, including those of the Khmers and Malays, have flourished there. In the late 15th cent., Islamic influences grew strong but were overshadowed by the arrival of Europeans, who established their power throughout Southeast Asia; only Thailand remained free of colonial occupation. Because of Southeast Asia's strategic location between Japan and India, and the importance of shipping routes that traverse it, the region became the scene of battles between Allied and Japanese forces during World War II.

After the war the countries of Southeast Asia have reemerged as independent nations. They have been plagued by political turmoil, weak economies, ethnic strife, and social inequities, although the situation for most Southeast Asian nations improved in the 1980s and 90s. Throughout the 1960s and early 1970s, however, there were open conflicts between Communist and non-Communist factions throughout most of the region, especially in Vietnam, Laos, and Cambodia (see Vietnam War). In 1967 Indonesia, Malaysia, the Philippines, Singapore, and Thailand created the Association of Southeast Asian Nations (ASEAN), the objectives of which are to promote regional economic growth, political stability, social progress, and cultural developments. Since then, Brunei (1984), Vietnam (1995), Laos (1997), and Myanmar and Cambodia (1999) have joined ASEAN. In 1997 a monetary collapse in Thailand sparked a general economic crisis in several nations in the region; the results were most severe in Indonesia, which underwent economic, political, and social turmoil in the late 1990s.