The control of ribosome synthesis has been a major focus in molecular biology for over 50 years. As protein synthesis is an essential, yet energetically costly, process, all cells (from bacteria to mammals) devote complex regulatory networks to fine-tune the expression of ribosomal RNA (and therefore ribosome synthesis) to the nutritional environment. In Escherichia coli, ribosomal RNA promoters are among the strongest in the cell and are regulated by trans-acting proteins (Fis and H-NS) and small molecules (guanosine 5'-diphosphate 3'-diphosphate and initiating nucleoside triphosphates). Recent work has dissected many of the molecular mechanisms responsible for the strength and regulation of rRNA promoters.