The aim of this paper is threefold: first, to introduce the topic of youth slang by giving an overview of its main characteristics; second, to show the different word-formation processes that slang has to make the speaker's message more expressive; and third, to study the extent to which these two aspects are reflected in two corpora representing London and Madrid youth language. The present study is based, primarily, on an inventory of the top ten 'proper' and 'dirty' slang words in each language variety with particular emphasis on the speakers' age and gender, and, secondarily, on the entire corpus data, which showed great agreement with the features outlined in the overview of the main characteristics of youth slang, while the most obvious word-formation mechanisms turned out to be related to change of form and change of meaning.