This article discusses elements of criminal vocabulary (criminal dialect) that appear in the statements of people serving prison sentences in Poland and Great Britain. A fundamental difficulty in the process of researching criminal vocabulary is access to reliable sources of research material. The literature on the subject points in particular to the lack of studies in the given area. In the case of the prison environment, some characteristic words or phrases of the criminal subculture may penetrate into the colloquial language. Some, especially young people, fascinated by criminal life, the underworld, pride themselves on knowing the criminal dialect and are freely using it. Phrases of the prison dialect often appear in musical works performed by rappers, which testifies to its pervasion behind the "prison walls".