This article concerns the causes of the transition from slang lexical units of criminal gangs to a more open form of language – youth slang. The author in her study also lists the basic processes accompanying the mentioned transition in the Spanish language – fluctuation of phonemes, apocope, truncation and deformation of lexical units, productive mechanisms that change lexical values using the metonymical and metaphorical transfer, contraction and expansion of the lexical and semantic values of the units. Lexical units from different language formas are compared on the basis of the thesaurus compiled by the Spanish Forensic linguist S. Garcia Ramos, and five dictionaries of the Spanish youth slang. The article may be of interest to students and professionals in Spanish studies, Sociolinguistics, Spoken language forms and Forensic Linguistics.