The cap structure that is characteristic of all polymerase-II-transcribed RNAs has been shown to play an important role in many aspects of RNA metabolism including RNA processing, RNA nuclear transport, and translation initiation. The effects of the cap structure on these different processes is mediated by proteins that recognise and bind to it, and are therefore generically called cap-binding proteins. For example, the cap-binding protein eIF4E, in a complex with other proteins, mediates the effect of the cap on the initiation of translation. EIF-4E is predominantly localised in the cytoplasm. In the last five years, it has been demonstrated that a second cap-binding protein complex, which is mainly localised in the nucleus, mediates the stimulatory effects of the cap in nuclear processes such as pre-mRNA splicing, RNA 3'-end formation, and RNA nuclear export. The purpose of this review is to summarise our current knowledge on the role of the cap structure and of the cap-binding protein complex in nuclear RNA metabolism and present evidence that at least some processes may be coupled in vivo.