A dialect continuum or dialect chain is a series of language varieties spoken across some geographical area such that neighboring varieties are mutually intelligible, but the differences accumulate over distance so that widely separated varieties may not be. This is a typical occurrence with widely spread languages and language families around the world, when these languages did not spread recently. Some prominent examples include the Indo-Aryan languages across large parts of India, varieties of Arabic across north Africa and southwest Asia, the Turkic languages, the varieties of Chinese, and parts of the Romance, Germanic and Slavic families in Europe. Terms used in older literature include dialect area (Leonard Bloomfield) and L-complex (Charles F. Hockett).

Dialect continua typically occur in long-settled agrarian populations, as innovations spread from their various points of origin as waves. In this situation, hierarchical classifications of varieties are impractical. Instead, dialectologists map variation of various language features across a dialect continuum, drawing lines called isoglosses between areas that differ with respect to some feature.

A variety within a dialect continuum may be developed and codified as a standard language, and then serve as an authority for part of the continuum, e.g. within a particular political unit or geographical area. Since the early 20th century, the increasing dominance of nation-states and their standard languages has been steadily eliminating the nonstandard dialects that comprise dialect continua, making the boundaries ever more abrupt and well-defined.

Dialectologists record variation across a dialect continuum using maps of various features collected in a linguistic atlas, beginning with an atlas of German dialects by Georg Wenker (from 1888), based on a postal survey of schoolmasters. The influential Atlas linguistique de la France (1902–10) pioneered the use of a trained fieldworker. These atlases typically consist of display maps, each showing local forms of a particular item at the survey locations.

Secondary studies may include interpretive maps, showing the areal distribution of various variants. A common tool in these maps is an isogloss, a line separating areas where different variants of a particular feature predominate.

In a dialect continuum, isoglosses for different features are typically spread out, reflecting the gradual transition between varieties. A bundle of coinciding isoglosses indicates a stronger dialect boundary, as might occur at geographical obstacles or long-standing political boundaries. In other cases, intersecting isoglosses and more complex patterns are found.

Standard varieties may be developed and codified at one or more locations in a continuum until they have independent cultural status (autonomy), a process the German linguist Heinz Kloss called ausbau. Speakers of local varieties typically read and write a related standard variety, use it for official purposes, hear it on radio and television, and consider it the standard form of their speech, so that any standardizing changes in their speech are towards that variety. In such cases the local variety is said to be dependent on, or heteronomous with respect to, the standard variety.

A standard variety together with its dependent varieties is commonly considered a "language", with the dependent varieties called "dialects" of the language, even if the standard is mutually intelligible with another standard from the same continuum. The Scandinavian languages, Danish, Norwegian and Swedish, are often cited as examples. Conversely, a language defined in this way may include local varieties that are mutually unintelligible, such as the German dialects.