In-company training undertaken by vocational students can be a very good opportunity to acquire skills; but it is also a very delicate process. The author, who examined the traineeships of students in formal vocational training in Catalonia as part of his doctoral thesis in economics, understands skills acquisition as an interactive process between the law, the behaviour of the different actors (trainees, schools and companies) and the pressure exerted by the labour market. This article describes skills acquisition in Catalonia and backs its description with quantitative and qualitative data allowing a better understanding of the interaction between the various factors and actors. (Contains 2 tables.)