Language shift, also known as language transfer, language replacement or language assimilation, is the process whereby a speech community shifts to a different language, usually over an extended period of time. Often, languages that are perceived to be of higher-status stabilize or spread at the expense of other languages that are perceived—even by their own speakers—to have lower status. An example is the shift from Gaulish to Latin during the time of the Roman Empire.

Language assimilation may operate alongside other aspects of cultural assimilation when different cultures meet and merge.

For prehistory, Forster et al. (2004) and Forster and Renfrew (2011) observe that there is a correlation of language shift with intrusive male Y chromosomes but not necessarily with intrusive female mtDNA. They conclude that technological innovation (the transition from hunting-gathering to farming, or from stone to metal tools) or military prowess (as in the abduction of British women by Vikings to Iceland) causes immigration of at least some men, who are perceived to be of higher status than local men. Then, in mixed-language marriages, children would speak the "higher-status" language, yielding the language/Y-chromosome correlation seen today.

Assimilation is the process whereby a speech-community becomes bilingual and gradually shifts allegiance to the second language. The rate of assimilation is the percentage of the speech-community that speaks the second language more often at home. The data are used to measure the use of a given language in the lifetime of a person, or most often across generations. When a speech-community ceases to use their original language, language death is said to occur.

In the context of the Indo-European migrations, it has been noted that small groups can change a larger cultural area. Michael Witzel refers to Ehret's model "which stresses the osmosis, or a "billiard ball", or Mallory's Kulturkugel, effect of cultural transmission." According to Ehret, ethnicity and language can shift with relative ease in small societies, due to the cultural, economic and military choices made by the local population in question. The group bringing new traits may initially be small, contributing features that can be fewer in number than those of the already local culture. The emerging combined group may then initiate a recurrent, expansionist process of ethnic and language shift.

David Anthony notes that the spread of the Indo-European languages probably did not happen through "chain-type folk migrations", but by the introduction of these languages by ritual and political elites, which are emulated by large groups of people. Anthony explains:

Language shift can be understood best as a social strategy through which individuals and groups compete for positions of prestige, power, and domestic security ... What is important, then, is not just dominance, but vertical social mobility and a linkage between language and access to positions of prestige and power ... A relatively small immigrant elite population can encourage widespread language shift among numerically dominant indigenes in a non-state or pre-state context if the elite employs a specific combination of encouragements and punishments. Ethnohistorical cases ... demonstrate that small elite groups have successfully imposed their languages in non-state situations.

Anthony gives the example of the Luo-speaking Acholi in northern Uganda in the 17th and 18th century, whose language spread rapidly in the 19th century. Anthony notes that "Indo-European languages probably spread in a similar way among the tribal societies of prehistoric Europe", carried forward by "Indo-European chiefs" and their "ideology of political clientage". Anthony notes that "elite recruitment" may be a suitable term for this system.