Abstract: Mediator, a macromolecular complex comprising approximately 20 different protein components, is largely responsible for the tight control of transcription that underpins cell development, differentiation, and maintenance in eukaryotes from yeast to human. In the past five years, macromolecular electron microscopy has been used to characterize the structure of Mediator, and of the complexes it forms with other components of the transcription machinery. The results reveal how Mediator interacts with RNA polymerase II, and suggest that regulatory information could be conveyed through changes in Mediator conformation that would influence the transcription initiation process.