The problem of the so-called 'false friends' has raised little interest within Spanish contrastive lexicography and lexicology, especially as to the theoretical aspects. For this reason, the purpose of this paper is to offer a certain theoretical basis in order to establish a classification of the different types of false friends. A multidisciplinary approach is required for the study of false friends, which can be situated mainly within the frame of contrastive lexicology. There can be two main types of false friends: formal and semantic. Therefore, for the classification of false friends both the expression level and the content level should be considered, as well as the existence of false friends in the field of phraseology. Furthermore, the article exposes some coincidences and divergences between the Spanish and the German lexicon.