The large number of ethnic and linguistic groups, the population composition, and the peopling process in the state has led to it being called an "India in miniature".

These classifications are now considered to have little validity, and they yield inconsistent results; the current understanding is based on ethnolinguistic groups[3] and in consonance with genetic studies.

Geographically Assam, in the middle of Northeast India, contains fertile river valleys surrounded and interspersed by mountains and hills.

[48] The arrival of the Indo-Aryans and the expansion of the Kamarupa kingdom over the entire Brahmaputra valley created the conditions for the creolisation and development of proto-Boro-Garo lingua franca.

[54] Eye witness accounts of the Austroasiatic and Tibeto-Burman peoples come from the Periplus of the Erythraean Sea (1st century CE) and Ptolemy's Geography (2nd century CE) that call the land Kirrhadia after the Indo-Aryan name for the non-Indo-Aryan Kirata people who were the source of Malabathrum, so priced in the classical world.

[64] In the period when Indo-Aryan settlements were being created, Kamarupa likely constituted urban centers along the Brahmaputra river in which a precursor of the Assamese language was spoken with Austroasiatic and Tibeto-Burman communities everywhere else.

[67] The fourth stream of new arrivals were Muslim personnel of the army of Muhammad Bin Bakhtiyar Khalji left back after his disastrous Tibet expeditions.

The fifth wave of immigrants were Tai Shan People, who entered Assam under the leadership of Sukaphaa from Hukawng Valley in Myanmar[69] via Pangsau Pass in 1228 and settled between Buridihing and Dikhou rivers.

They assimilated some of the Naga, Moran, Borahi, Chutiya and Dimasa peoples in a process of Ahomisation till they themselves began to be Hinduized from the mid-16th century onwards.

They constitute the Singphos in Upper Assam, and the Kuki-Chin tribes in Karbi Anglong, Dima Hasao and Barak Valley.

[76] Following the establishment of the tea industry in Assam, and after the companies failed in harnessing the labour of the local Kachari, people from the Chotanagpur area of Bihar, northern and western Orissa, eastern Madhya Pradesh, and northern Andhra Pradesh belonging to Munda, Ho, Santal, Savara, Oraon, Gond and other ethnic groups were recruited for labour in the newly emerging tea estates.

[79] British colonialism opened the borders of Assam, hitherto controlled tightly by the Ahom and Dimasa kingdoms, and established a new order[80] causing a significant influx from Bengal, Rajasthan, North India and Nepal.

[84] In the 19th century the peasant economy was completely in their grip and Marwari traders also participated as bankers and commercial agents of the nascent Assam Tea industry.

[87] In the first two decades of the 20th century the colonial government encouraged Newar and other ethnic non-Bahun Nepali communities to settle in Assam's excluded areas mostly as "professional" cattle grazers for an expanding revenue,[88] feeding into the business of milk supply in the emerging urban markets.

[89] Muslim landless cultivators from Mymensingh in present-day Bangladesh, encouraged by the landlords of Goalpara and the British administration, began arriving in the late 19th century seeking land.

[90] The initial trickle showed dramatic increases in each succeeding decade after 1901—by 1911, the Mymensingh cultivators were joined by lesser numbers from Pabna, Bogra and Rangpur who settled in the Char lands of Goalpara and some beyond;[91] by 1921 the immigrants were settled up to the central districts of Assam, mostly along the Brahmaputra though many had ventured further away, with some close to the Bhutan border;[92] and by 1931 the increases have been so dramatic that even British officers began talking about demographic shifts.

[94] The Partition of India triggered an exodus of Bengali Hindu people mostly from Sylhet Division in East Pakistan to Assam, numbering between 700 and 800 thousands.

Unlike the Muslim cultivator who came from Mymensingh and from the west seeking land, this group came in from the south and settled mostly around towns, service centers and railway stations.

Collectively called desuwali (a local corruption of deshwali referring to their homeland or desh), they came from marginal socioeconomic backgrounds and came as construction workers, handcart-pullers, rickshaw pullers, cobblers, barbers and eventually settled in Assam, usually in interior fallow lands—and many of them found reasonable successes in trade and commerce and have become politically assertive.

Tribal groupings migrated to the soils of Assam from diverse directions, as the territory was linked to a number of states and many different countries throughout history.

Austro-Asiatics, Tibeto-Burmans, and Indo-Aryans are historically the most important and oldest traditional groups to have arrived in Assam, and to this day, they remain essential elements of the "Assamese Diaspora".

Along with the Tai Ahoms, other prominent groups ruled parts of the Assam valley during the medieval period, notably the Chutias, Kochs, and Dimasas.