In this article, I will discuss different types of lexical co-occurrences and examine the requirements for representing them in a reusable lexical resource. I will focus the discussion on the delimitation of a limited set of descriptive parameters rather than on an exhaustive classification of idioms or multiword units. Descriptive parameters will be derived from a detailed discussion of the problem of how to determine adequate translations for such units. Criteria for determining translation equivalences between multiword units of two languages will be: the syntactic and the semantic structure as well as functional, pragmatic, and stylistic properties.