The Celts (pronounced /ˈkɛlts/ or /ˈsɛlts/, see names of the Celts) were a diverse group of tribal societies in Iron Age and Roman-era Europe who spoke Celtic languages.

The earliest archaeological culture commonly accepted as Celtic, or rather Proto-Celtic, was the central European Hallstatt culture (ca. 800-450 BC), named for the rich grave finds in Hallstatt, Austria. By the later La Tène period (ca. 450 BC up to the Roman conquest), this Celtic culture had expanded over a wide range of regions, whether by diffusion or migration: to the British Isles (Insular Celts), the Iberian Peninsula (Celtiberians, Celtici ), much of Central Europe, (Gauls) and following the Gallic invasion of the Balkans in 279 BC as far east as central Anatolia (Galatians).

The earliest directly attested examples of a Celtic language are the Lepontic inscriptions, beginning from the 6th century BC. Continental Celtic languages are attested only in inscriptions and place-names. Insular Celtic is attested from about the 4th century AD in ogham inscriptions, although it is clearly much earlier. Literary tradition begins with Old Irish from about the 8th century. Coherent texts of Early Irish literature, such as the Táin Bó Cúailnge (The Cattle Raid of Cooley), survive in 12th-century recensions. According to the theory of John T. Koch and others, the Tartessian language may have been the earliest directly attested Celtic language with the Tartessian written script used in the inscriptions based on a version of a Phoenician script in use around 825 BC.

By the early 1st millennium AD, following the expansion of the Roman Empire and the Great Migrations (Migration Period) of Germanic peoples, Celtic culture had become restricted to the British Isles (Insular Celtic), and the Continental Celtic languages ceased to be widely used by the 6th century.

Insular Celtic culture diversified into that of the Gaels, the Welsh and the Bretons of the medieval and modern periods. A modern "Celtic identity" was constructed in the context of the Romanticist Celtic Revival, mostly in Great Britain and Ireland.