The purpose of this paper is to give evidence on the role played by metonymy in grammar and in language use. In fact, metonymy motivates the use of specific grammatical structures and underlies the conventional elements in certain grammatical constructions (understood as pairings of form and meaning). The authors also pay attention to how some grammatical phenomena constrain a number of metonymic operations, thus giving rise to a two-way relationship between metonymy and grammar. As an illustration of the grammatical phenomena on which metonymy places constraints, the authors discuss different forms of categorial and subcategorial conversion, and anaphora. Furthermore, the authors argue that generic idealized cognitive models (ICMs) are found at the basis of metonymic mappings that have grammatical effects. They claim that in order to characterize the full meaning impact of certain constructions we have to take into consideration the metonymic structures that underlie them. The paper also provides an exhaustive characterization of metonymy in relation to the degrees of centrality of the semantic features in a domain, the number of correspondences in a mapping, and the two basic types of domain inclusion. The analysis is fully based on English expressions. (Elena Ruiz Gil and Francisco Ruiz de Mendoza)