Mediator is an evolutionarily conserved multisubunit RNA polymerase II (Pol II) coregulatory complex. Although Mediator was initially found to play a critical role in the regulation of the initiation of Pol II transcription, recent studies have brought to light an expanded role for Mediator at post-initiation stages of transcription.We provide a brief description of the structure of Mediator and its function in the regulation of Pol II transcription initiation, and we summarize recent findings implicating Mediator in the regulation of various stages of Pol II transcription elongation.Emerging evidence is revealing new roles for Mediator in nearly all stages of Pol II transcription, including initiation, promoter escape, elongation, pre-mRNA processing, and termination.Mediator plays a central role in the regulation of gene expression by impacting nearly all stages of mRNA synthesis. This article is part of a Special Issue entitled: RNA polymerase II Transcript Elongation.