Synthesis of ribosomal RNA by RNA polymerase (Pol) I is the first step in ribosome biogenesis and a regulatory switch in eukaryotic cell growth. In this thesis a reproducible large-scale purification protocol for Pol I from S. cerevisiae could be developed. Crystals were obtained, diffraction to < 4 Å could be recorded, however, the enormously complex non-crystallographic symmetry impeded structure solution. Switching to cryo-electron microscopy, the structure of the complete 14-subunit enzyme could be solved to 12 Å resolution, a homology model for the core enzyme could be generated, and the crystal structure of the subcomplex A14/43 could be solved. In the resulting hybrid structure of Pol I, A14/43, the clamp, and the dock domain contribute to a unique surface interacting with promoter-specific initiation factors. The Pol I-specific subunits A49 and A34.5 form a heterodimer near the enzyme funnel that acts as a built-in elongation factor, and is related to the Pol II-associated factor TFIIF. In contrast to Pol II, Pol I has a strong intrinsic 3’-RNA cleavage activity, which requires the C-terminal domain of subunit A12.2, and apparently enables rRNA proofreading and 3’-end trimming.