The Cornish language (in Cornish: Kernowek, Kernewek, Curnoack) is one of the Brythonic group of Celtic languages which includes Welsh, Breton and, originally, Cumbrian. The Celtic languages of Scottish Gaelic, Irish and Manx are known as part of the separate Goidelic group.

In terms of similarity of Cornish to the other existing Celtic languages, it shares about 80% basic vocabulary with Breton, 75% with Welsh, 35% with Irish and 35% with Scots Gaelic. Welsh shares about 70% with Breton.

Early Modern Cornish was the subject of a study by the Welsh linguist Edward Lhuyd in 1700, and differs from the mediaeval language in having a simpler structure and grammar. By this time the language was already arguably in decline from its earlier heyday, and the situation worsened over the course of the next century. It is often claimed that the last native speaker of Cornish was the Mousehole resident Dolly Pentreath, who died in 1777. Since she spoke at least some English, Pentreath was certainly not the last monoglot Cornish speaker; that is believed to be Chesten Marchant, who died in 1676 at Gwithian. It does, however, appear to be true that Dolly Pentreath spoke Cornish fluently and was probably the last to do so prior to the 20th century revival of the language. There is evidence that Cornish continued, albeit in limited usage by a handful of speakers, throughout the 19th century and into the early 20th century. In 1875 six speakers all in their sixties were discovered. Fishermen, for example, were counting fish in the Cornish language into the 1940s. Some dialects of English spoken in Cornwall display strong influences from the Cornish language, which almost certainly go back several centuries.

In the 20th century a conscious effort was made to revive Cornish as a language for everyday use in speech and writing (see below for further details about the dialects of modern Cornish).

There are regular periodicals solely in the language: An Gannas, An Gowser and An Garrick. Radio Cornwall has regular news broadcasts in Cornish, and sometimes other programmes and features for learners and enthusiasts. Local newspapers such as the Western Morning News regularly have articles in Cornish, and newspapers such as The Packet, The West Briton and The Cornishman also support the movement.

The language has financial sponsorship from many sources, including the Millennium Commission. Increasingly, churches have notices in Cornish and English. The take-up of the language is now becoming so widespread that organisations such as Kesva an Taves Kernewek, the Cornish Language Board, are finding it difficult to keep up with demand. Others include the Cornish sub-group of the European bureau for lesser-used languages, Teere ha Tavas, or land and language, Gorseth Kernow, Cowethas an Yeth, Agan Tavas and Dalleth, the last of which is the organisation promoting language to pre-school children. There are many popular ceremonies, some ancient, some modern, which use the language or are entirely in the language.

Cornwall has many other cultural events associated with the language, including the prestigious international Celtic film festival, hosted in St Ives in 1997, with the programme in Cornish, English and French. There have been many films, some televised, made entirely, or significantly, in the language. Some shops, such as Gwynn ha Du, the Cornish book shop in the town of Liskeard, sell books written in Cornish. Many companies use Cornish names. The GP overnight service in Cornwall is now called Kernowdoc. Cornish is taught in some schools and there are many who study Cornish at degree level in Aberystwyth and Harvard, USA.

The Cornish language finally has an edition of the New Testament in Kernowek. Nicholas Williams' translation of the Testament Noweth agan Arluth ha Savyour Jesu Cryst was published at Easter 2002 (ISBN 0-9535975-4-7); it uses Unified Cornish Revised orthography. A complete Bible has been translated and is to be published in 2004.