The nucleolus is the site within the eukaryotic nucleus of transcription of rDNA, of processing of the rDNA transcripts, and of the formation of pre-ribosomal particles. We review current ideas for the molecular organization of these processes. The earliest transcriptional events take place near the junction of the fibrillar centers and the dense fibrillar component; nascent transcripts occupy the dense fibrillar component, and the later stages of pre-ribosome formation take place in the granular component. We review current knowledge of non-ribosomal nucleolar proteins. Nucleoli contain a group of proteins that bind RNA and are likely to act as chaperones to facilitate the molecular structural events in the folding and interaction of the many components of ribosomes. Some of these nucleolar proteins are also engaged in a shuttling cycle between the nucleus and the cytoplasm and may serve to transport other proteins.