This chapter discusses the mechanisms of eukaryotic ribosomal transcription and its regulation. Ribosomal transcription has been studied in an exceptionally wide range of organisms. In eukaryotes, ribosomal gene transcription uses a dedicated set of transcription factors and a specialized ribonucleic acid (RNA) polymerase, the deoxyribonucleic acid (DNA)-dependent RNA polymerase I (RPOI). The resultant precursor-rRNA (pre-rRNA) is neither capped nor polyadenylated and is produced in the nucleolus; a large nuclear structure visible in the light microscope. Moreover, the promotion of ribosomal transcription is generally directed, by a 100- to 150-bp DNA sequence that lies across and, predominantly, upstream of the transcription-initiation site. Promotion requires an activated form of RPOI, the DNA-binding factor upstream transcription factor (UBF), and the TATA binding protein (TBP) I -complex, containing three TATA-box binding protein associated protein (TAF) I s. Upstream binding factor (UBF) binds to both an upstream and an initiation-site promoter element, probably coiling the promoter into a 180-bp loop. This allows the interaction of the TBP,-complex, and subsequent polymerase recruitment. The activity of the promoter is enhanced by a promoter-proximal terminator, by enhancer repeats in the proximal IGS, and by the presence in the IGS of duplicate spacer promoters. Further upstream, other sequences may also modulate transcription.