Sociolinguistics is the descriptive, scientific study of how language is shaped by, and used differently within, any given society. The field largely looks at how a language varies between distinct social groups and under the influence of assorted cultural norms, expectations, and contexts, including how that variation plays a role in language change. Sociolinguistics combines the older field of dialectology with the social sciences in order to identify regional dialects, sociolects, ethnolects, and other sub-varieties and styles within a language.

A major branch of linguistics since the second half of the 20th century, sociolinguistics is closely related to and can partly overlap with pragmatics, linguistic anthropology, and sociology of language, the latter focusing on the effect of language back on society. Sociolinguistics' historical interrelation with anthropology can be observed in studies of how language varieties differ between groups separated by social variables (e.g., ethnicity, religion, status, gender, level of education, age, etc.) or geographical barriers (a mountain range, a desert, a river, etc.). Such studies also examine how such differences in usage and in beliefs about usage produce and reflect social or socioeconomic classes. As the usage of a language varies from place to place, language usage also varies among social classes, and some sociolinguists study these sociolects.

Studies in the field of sociolinguistics use a variety of research methods including ethnography and participant observation, analysis of audio or video recordings of real life encounters or interviews with members of a population of interest. Some sociolinguists assess the realization of social and linguistic variables in the resulting speech corpus. Other research methods in sociolinguistics include matched-guise tests (in which listeners share their evaluations of linguistic features they hear), dialect surveys, and analysis of preexisting corpora.

The social aspects of language were in the modern sense first studied by Indian and Japanese linguists in the 1930s, and also by forerunners in Denmark and Switzerland around the turn of the 20th century, but none received much attention in the West until much later. The study of the social motivation of language change, on the other hand, has its foundation in the wave model of the late 19th century. The first attested use of the term sociolinguistics was by Thomas Callan Hodson in the title of his 1939 article "Sociolinguistics in India" published in Man in India.

Dialectology is an old field, and in the early 20th century, dialectologists such as Hans Kurath and Raven I. McDavid Jr. initiated large scale surveys of dialect regions in the U.S.

The study of sociolinguistics in the West was pioneered by linguists such as Charles A. Ferguson or William Labov in the US and Basil Bernstein in the UK. In the 1960s, William Stewart and Heinz Kloss introduced the basic concepts for the sociolinguistic theory of pluricentric languages, which describes how standard language varieties differ between nations, e.g. regional varieties of English versus pluricentric "English"; regional standards of German versus pluricentric "German"; Bosnian, Croatian, Montenegrin, and Serbian versus pluricentric "Serbo-Croatian". Dell Hymes, one of the founders of linguistic anthropology, is credited with developing an ethnography-based sociolinguistics and is the founder of the journal Language in Society. His focus on ethnography and communicative competence contributed to his development of the SPEAKING method: an acronym for setting, participants, ends, act sequence, keys, instrumentalities, norms, and genres that is widely recognized as a tool to analyze speech events in their cultural context.

Sociolinguistics can be divided into subfields, which make use of different research methods, and have different goals. Dialectologists survey people through interviews, and compile maps. Ethnographers such as Dell Hymes and his students often live amongst the people they are studying. Conversation analysts such as Harvey Sacks and interactional sociolinguists such as John J. Gumperz record audio or video of natural encounters, and then analyze the tapes in detail. Sociolinguists tend to be aware of how the act of interviewing might affect the answers given.

Some sociolinguists study language on a national level among large populations to find out how language is used as a social institution. William Labov, a Harvard and Columbia University graduate, is often regarded as the founder of variationist sociolinguistics which focuses on the quantitative analysis of variation and change within languages, making sociolinguistics a scientific discipline.