This article discusses the treatment of collocations in the context of a long-term project on the development of multilingual NLP tools. Besides "classical" two-word collocations, we will focus on the case of complex collocations (3 words or more) for which a recursive design is presented in the form of collocation of collocations. Although comparatively less numerous than two-word collocations, the complex collocations pose important challenges for NLP. The article discusses how these collocations are retrieved from corpora, inserted and stored in a lexical database, how the parser uses such knowledge and what are the advantages offered by a recursive approach to complex collocations.