Publisher Summary This chapter discusses the structure and function of RNA polymerase II. High-resolution structural studies of polymerase II (Pol II) by x-ray crystallography required large amounts of pure protein that cannot be obtained by over-expression because of the complexity of the enzyme. The described structural studies of yeast Pol II are directly relevant to Pol II enzymes in higher organisms, because Pol II subunits are very well conserved in sequence and function. After successful RNA chain elongation, transcription terminates and Pol II dissociates from the template. Some of the steps during the transcription cycle can be carried out by Pol II alone. Pol II can maintain an open transcription bubble, translocate along the template DNA, synthesize RNA from the template, and proofread the nascent RNA. However, for all other steps during the transcription cycle, Pol II requires additional proteins. The chapter presents backbone models of the complete initiation-competent Pol II, including the Rpb4/7 complex and the complete Pol II with bound elongation factor transcription factor IIS (TFIIS). The structures, together with many functional studies, have given many insights into the mechanism of mRNA transcription. Structural and functional studies of bacterial RNA polymerase allow for interesting comparisons and evolutionary considerations. The Pol II structures now guide mutagenesis experiments aimed at a dissection of the transcription mechanism.