The genes that code for 45S rRNA, the precursor of 18S, 5.8S and 28S rRNA, are transcribed by RNA polymerase I. In many eukaryotes the genes are arranged as tandem repeats in discrete chromosomal clusters. rDNA transcription and rRNA processing occur in the nucleolus. In vertebrates, at least two factors, SL-1 and UBF, specific for transcription by RNA polymerase I cooperate in the formation of the initiation complex. Interestingly, there are proteins analogous to SL-1 in unicellular eukaryotes, but the requirement for a UBF-like factor appears to vary. Recent advances in our understanding of the rDNA transcription system and its regulation have demonstrated overlap with the other nuclear transcription systems (RNA polymerase II and III). This is exemplified by the utilization of TBP as a component of SL-1 and the role of Rb in regulatory rDNA transcription.