Multi-word Lexical Units (MWLU) are of great importance in language in general, and in Natural Language Processing in particular, since they are not governed by the free rules of the system. In this article, we give an overview of the different types of phraseological units, explaining briefly each one's features. Our priority being to process idioms automatically in Basque texts, we concisely analyze several approaches for the inflectional description of MWLUs, and then, we explain the system we have developed for Basque: (i) a general representation for describing MWLUs in the lexical database for Basque (EDBL), (ii) HABIL, a tool capable of detecting and analyzing them based on the features described in the database, and (iii) a constraint grammar for disambiguating ambiguous MWLUs.