In phonology, assimilation is a sound change in which some phonemes (typically consonants or vowels) change to become more similar to other nearby sounds. This process is common across languages and can happen within a word or between words. For example, in English "handbag" (/ˈhændbæɡ/), the [n] often shifts to [m] in rapid speech, becoming /ˈhæmbæɡ/, because [m] and [b] are both bilabial (produced with both lips), and their places of articulation are similar.

It occurs in normal speech but is more frequent in faster speech. Sometimes the change is accepted as canonical, and can even become recognized in standard spelling: implosion pronounced with [m], composed of in- + -plosion (as in explosion).

Sound segments typically assimilate to a following sound, but they may also assimilate to a preceding one. Assimilation most commonly occurs between immediately adjacent sounds but may occur between sounds separated by others. For example, in "handbag," the [d] is sometimes elided (omitted), which causes the [n] to assimilate to [m] before the [b].

Assimilation can be synchronic, an active process in a language at a given point in time, or diachronic, a historical sound change—for instance, "cupboard," historically a compound of "cup" (/kʌp/) and "board" (/bɔːrd/), is now pronounced /ˈkʌbərd/, with the original /ˈkʌpbɔːrd/ almost never used.

A related process, coarticulation, includes changes like vowels nasalizing (taking on a nasal sound) before nasal consonants (/n, m, ŋ/) due to premature soft palate (velum) lowering, or /b/ labializing (lips rounding) as in "boot" ([bʷuːt̚]) or "ball" [bʷɔːɫ] in some accents. This article describes both processes under the term assimilation.

The physiological or psychological mechanisms of coarticulation are unknown, and coarticulation is often loosely referred to as a segment being "triggered" by an assimilatory change in another segment. In assimilation, the phonological patterning of the language, discourse styles and accent are some of the factors contributing to changes observed.

There are four configurations found in assimilations:

Although all four occur, changes in regard to a following adjacent segment account for virtually all assimilatory changes and most of the regular ones.^{[citation needed]} Assimilations to an adjacent segment are vastly more frequent than assimilations to a nonadjacent one. Those radical asymmetries might contain hints about the mechanisms involved, but they are not obvious.