Active rRNA transcription units and the RNA polymerase I transcription machinery are located within a well-defined transcription and processing "factory" within the nucleus called the nucleolus. Although sometimes referred to as an organelle, the nucleolus is not a true organelle because it is not surrounded by a membrane. When viewed by electron microscopy, the nucleolus has three distinct and conserved structural components. The first of these, the fibrillar center (FC), is a pale region that is enriched with RNA polymerase I and various transcription factors. A nucleolus may have many fibrillar centers. The second structural component, the dense fibrillar component (DFC), is a very dense fibrillar region that partially or completely surrounds the FC. The third structural component, the granular compartment (GC) occupies the rest of the nucleolus. This architecture has been proposed to reflect the vectorial maturation of the pre-ribosomes. According to one hypothesis, RNA synthesis takes piace at the interface between the FC and the DFC, with nascent transcripts extending into the body of the DFC and nascent pre-ribosomes moving from the DFC to the GC as the pre-rRNA is processed, modified, and assembled into a ribosome. The nucleolus is a dynamic structure that is dismantled during mitosis and reassembled at interphase. Its appearance provides clues to certain pathological conditions. For instance, cancer cells often have abnormally large nucleoli. With this background information in mind, we are now ready to examine the enzymes and protein factors involved in pre-RNA formation.