There have been many theories proposing the origin of the Tai peoples — of which the Lao are a subgroup. The Chinese Han Dynasty chronicles of the southern military campaigns provide the first written accounts of Tai–Kadai speaking peoples who inhabited the areas of modern Yunnan China and Guangxi.

James R. Chamberlain (2016) proposes that Tai-Kadai (Kra-Dai) language family was formed as early as the 12th century BCE in the middle Yangtze basin, coinciding roughly with the establishment of the Chu and the beginning of the Zhou dynasty.^[9] Following the southward migrations of Kra and Hlai (Rei/Li) peoples around the 8th century BCE, the Be-Tai people started to break away to the east coast in the present-day Zhejiang, in the 6th century BCE, forming the state of Yue.^[9] After the destruction of the state of Yue by Chu army around 333 BCE, Yue people (Be-Tai) began to migrate southwards along the east coast of China to what are now Guangxi, Guizhou and northern Vietnam, forming Luo Yue (Central-Southwestern Tai) and Xi Ou (Northern Tai).^[9] The Tai peoples, from Guangxi and northern Vietnam began moving south – and westwards in the first millennium CE, eventually spreading across the whole of mainland Southeast Asia.^[10] Based on layers of Chinese loanwords in proto-Southwestern Tai and other historical evidence, Pittayawat Pittayaporn (2014) proposes that the southwestward migration of Tai-speaking tribes from the modern Guangxi and northern Vietnam to the mainland of Southeast Asia must have taken place sometime between the 8th–10th centuries.^[11] Tai speaking tribes migrated southwestward along the rivers and over the lower passes into Southeast Asia, perhaps prompted by the Chinese expansion and suppression. A 2016 mitochondrial genome mapping of Thai and Lao populations supports the idea that both ethnicities originate from the Tai–Kadai (TK) language family.^[12]

The Tai, from their new home in Southeast Asia, were influenced by the Khmer and the Mon and most importantly Buddhist India. The Tai Kingdom of Lanna was founded in 1259. The Sukhothai Kingdom was founded in 1279 and expanded eastward to take the city of Chantaburi and renamed it to Vieng Chan Vieng Kham (modern Vientiane) and northward to the city of Muang Sua which was taken in 1271 and renamed the city to Xieng Dong Xieng Thong or "City of Flame Trees beside the River Dong", (modern Luang Prabang, Laos). The Tai peoples had firmly established control in areas to the northeast of the declining Khmer Empire. Following the death of the Sukhothai king Ram Khamhaeng, and internal disputes within the kingdom of Lanna, both Vieng Chan Vieng Kham (Vientiane) and Xieng Dong Xieng Thong (Luang Prabang) were independent city-states until the founding of the kingdom of Lan Xang in 1354.^[13]

The history of the Tai migrations into Laos were preserved in myth and legends. The Nithan Khun Borom or "Story of Khun Borom" recalls the origin myths of the Lao, and follows the exploits of his seven sons to found the Tai kingdoms of Southeast Asia. The myths also recorded the laws of Khun Borom, which set the basis of common law and identity among the Lao. Among the Khamu the exploits of their folk hero Thao Hung are recounted in the Thao Hung Thao Cheuang epic, which dramatizes the struggles of the indigenous peoples with the influx of Tai during the migration period. In later centuries the Lao themselves would preserve the legend in written form, becoming one of the great literary treasures of Laos and one of the few depictions of life in Southeast Asia prior to Therevada Buddhism and Tai cultural influence.^[14]