This final paper focuses on the language death of Celtic languages, primarily caused by the English language, on the territory of Great Britain and the United Kingdom. In the first part of the work, the terms “language death“ and “language shift“ are explained. These are the processes that many languages in the world face or have faced in the past, among which are Celtic languages as well presented in this paper in the second part of the work. Therefore, in the second part of the work, the decline in the use of Insular Celtic languages and various reasons which caused it are elaborated. The Insular Celtic group is divided into two branches, the Goidelic branch where Irish, Scottish Gaelic and Manx are placed; and the Brythonic branch where Welsh and Cornish are found. Irish, Scottish Gaelic and Welsh are extremely endangered, while Manx and Cornish could be considered dead since they are only marginally spoken. There are some revival attempts which up to this point have not been sufficient.