The C-terminal domain (CTD) of the large subunit of RNA polymerase (pol) II comprises conserved heptad repeats, and post-translational modification of the CTD regulates transcription and cotranscriptional RNA processing. Recently, the spatial patterns of modification of the CTD repeats have been investigated, and new functions of CTD modification have been revealed. In addition, there are new insights into the roles of the enzymes that decorate the CTD. We review these new findings and reassess the role of the pol II CTD in the regulation of gene expression.