To thrive, every living cell must continuously gauge and respond to changes in its environment. These changes are ultimately implemented by modulating gene expression, a process that relies on transcription by Nature's most multivalent molecular machine, the RNA polymerase. This book covers progress made over the past decade understanding how this machine functions to compute the cellular state, from the atomistic structural level responsible for chemistry to the integrative level at which RNA polymerase interacts with the other key molecular machineries of the cell.