The Lao kingdom of Lan Xang or Lan Ch'ang (Pali: Sisattanakhanahut, lâansâang - ລ້ານ "million" + ຊ້າງ "elephant", Vietnamese: Vạn Tượng) was established in 1354 by Fa Ngum.

Exiled as an infant to Cambodia, the Lao prince from Xieng Dong Xieng Thong married one of the Khmer king's daughters and in 1349 set out from Angkor at the head of a 10,000-man army. Fa Ngum organized the conquered principalities into provinces (muang), reclaimed Xieng Dong Xieng Thong from his father and elder brother and was crowned king of Lan Xang at Vientiane, the site of one of his victories (Victory of Phay Nam), in June 1354. The "million elephants" of the kingdom's name was an allusion to his formidable war machine. Lan Xang extended from the border of China to Sambor below the Mekong rapids at Khong Island and from the Vietnamese border to the western escarpment of the Khorat Plateau. It was one of the largest kingdoms in Southeast Asia. Muang Sua was the first kingdom established by the Tai.

The first few years of Fa Ngum's rule from his capital Xieng Dong Xieng Thong were uneventful. The next six years (1362-68), however, were troubled by religious conflict between Fa Ngum's lamaistic Buddhism and the region's traditional Theravada Buddhism. He severely repressed popular agitation that had anti-Mongol overtones and had many pagodas torn down. In 1368 Fa Ngum's Khmer wife died. He subsequently married the daughter of the king of Ayutthaya, who seems to have had a pacifying influence. For example, she was instrumental in welcoming a religious and artistic mission that brought with it a statue of the Buddha, the Phra Bang, from which the capital city was renamed Luang Prabang. It became the palladium of the kingdom. Popular resentment continued to build, however, and in 1373 Fa Ngum withdrew to Muang Nan. His son, Oun Heuan, who had been in exile in southern Yunnan, returned to assume the regency of the empire his father had created. Oun Heuan ascended to the throne as King Samsenethai (which means 300,000 Thai) in 1393 when his father died, ending Mongol overlordship of the middle Mekong Valley. Thai history records Samsenthai and all following rulers of Lan Xang as vassals paying tribute to Ayutthaya.

The kingdom, made up of Lao, Thai, and various ethnic hill tribes, lasted in its approximate borders for another 300 years and briefly reached an even greater extent in the northwest. Fa Ngum's descendants remained on the throne at Luang Prabang for almost 600 years after his death, maintaining the independence of Lan Xang to the end of the 17th century through a complex network of vassal relations with lesser princes. At the same time, these rulers fought off invasions from Vietnam (1478-79), Siam (1536), and Burma (1571-1621).

In 1694, however, Lan Xang fell prey to a series of rival pretenders to its throne, and, as a result of the ensuing struggles, split into three kingdoms in 1707: Luang Prabang, Vientiane, and Champasak which they each pay tributarie to Ayutthaya. Muang Phuan enjoyed a semi-independent status as a result of having been annexed by a Vietnamese army in the 15th century, an action that set a precedent for a tributary relationship with the court of Viet Nam at Hué.