Post-transcriptional regulation of gene expression is mediated by the interaction of protein factors with specific RNA sequences. In recent years, an increasing number of plant proteins that contain the principal RNA-binding domain, the RNA-recognition motif (RRM), have been identified. Many of these proteins can be classified into functional groups involved in different aspects of RNA metabolism. Each protein family has a characteristic domain structure, with one or more copies of the RRM and a variety of auxiliary domains. The most variable regions of the RRM of plant RNA-binding proteins probably contain determinants of target specificity, as has been shown for equivalent non-plant proteins. Thus, characterization of the RRM sequence of different plant RNA-binding proteins is likely to provide information about functional and/or evolutionary relationships.