This is a survey entry on mixed categories, including deverbal nominalizations, deverbal adjectivalizations and deadjectival nominalizations. Section 1 introduces the problems that mixed categories present for X' theory, and more specifically for the problem of headedness. Section 2 discusses derived and non‐derived deverbal nominalizations and shows how their analysis was first based on a distinction between syntax and lexicon, but later on formation in syntax only. In Section 3 the same is done for derived and non‐derived deverbal adjectivalizations. Section 4 focuses on deadjectival nominalizations, again with respect to their mixed properties. The article ends, in Section 5, with a short conclusion.