William Labov (b. 1927) is an American linguist who pioneered the study of variationist sociolinguistics. Born and raised in northern New Jersey, Labov studied English and philosophy at Harvard University (BA, 1948) and worked as an industrial chemist for several years before entering graduate school in linguistics at Columbia University in 1961. He completed his PhD in 1964, under the direction of Uriel Weinreich. He worked at Columbia until 1971, when he joined the faculty of the University of Pennsylvania, where he taught until his retirement in 2014. Labov’s influence on the field began with research he conducted in graduate school. His study of changing pronunciations on Martha’s Vineyard, the subject of his master’s thesis, introduced a method for observing sound change in progress and broke with tradition by exploring social motivations for linguistic innovations. For his PhD dissertation, Labov carried out a study of dialect patterns on the Lower East Side of New York City. Using a systematic, quantitative methodology, he demonstrated that linguistic variation is socially stratified, such that the use of pronunciation features (e.g., dropping of post-vocalic /r/) correlates with social class, ethnicity, etc. in regular patterns. Labov’s early research was greatly influential and inspired many scholars to carry out similar projects in other communities. The paradigm came to be known as variationist sociolinguistics. Much of Labov’s scholarship seeks to advance our understanding of language change. Historical linguists traditionally study completed linguistic changes, often long after they occurred, but Labov developed a method for examining active changes through a quantitative comparison of speakers representing several generations. This approach produces a new perspective on the change process by revealing intermediate stages. Labov has brought insights from this research to bear on theoretical debates within historical linguistics and the field more broadly. His work in this area has also documented many active sound changes in American English. Among these changes are innovations underway in particular dialects, such as the vowel changes in Philadelphia, as well as broader regional patterns, such as the Northern Cities Shift heard in the Great Lakes states. Throughout his career, social justice concerns have fueled Labov’s research. He has sought to demonstrate that the speech of stigmatized groups is as systematic and rule-governed as any other. He led a pioneering study in Harlem in the late 1960s that shone new light on African American English, demonstrating, for example, that grammatical usages like the deletion of the copula (e.g., He fast) are subject to regular constraints. Labov has served as an expert witness in court and before the U.S. Congress to share insights from his study of African American English. He has also worked to promote literacy for speakers of non-standard dialects, carrying out research on reading and developing material for the teaching of reading to these populations.