<p>Transcription elongation by RNA polymerase II (Pol II) is an integral step in eukaryotic gene expression. The speed of Pol II is controlled by a multitude of elongation factors, but the exact regulatory mechanisms remain incompletely understood, especially for higher eukaryotes. Here we develop a single-molecule platform to visualize the dynamics of individual mammalian transcription elongation complexes (ECs) reconstituted from purified proteins. This platform allows us to follow the elongation and pausing behavior of EC in real time and unambiguously determine the role of each elongation factor in the kinetic control of Pol II. We find that the mammalian EC harbors multiple speed gears dictated by its associated factors and phosphorylation status. Moreover, the elongation factors are not functionally redundant but act hierarchically and synergistically to achieve optimal elongation activity. We propose that such elaborate kinetic regulation underlies the major speed-changing events during the transcription cycle and enables cells to adapt to a changing environment.</p>