According to [5], the best way to formalize the notion of technical function is to treat such a notion as a family resemblance concept. A family resemblance concept (the term is due to L. Wittgenstein) is characterized and understood in terms of a network of similarity relations that link the various members of the class. Essential to an analysis of the notion of family resemblance is therefore a rigorous definition of the notion of similarity. We present two of the most authoritative formal approaches to similarity, the geometrical model and feature matching model, and we discuss the prospects of applying such models to the case of technical functions.