Small ubiquitin-related modifier (SUMO) is a protein moiety that is ligated to lysine residues in a variety of target proteins. The addition of SUMO can modulate the ability of proteins to interact with their partners, alter their patterns of subcellular localization and control their stability. It is clear that SUMO influences many different biological processes, but recent data suggest that it is particularly important in the regulation of transcription. Indeed, several transcription factors, such as Sp3, c-Jun, c-Myb and various nuclear receptors, have recently been shown to be subject to sumoylation and, although this modification can have a positive influence, a growing body of evidence highlights its role in the negative regulation of transcription. This review summarizes recent experiments focusing on sumoylation and transcriptional repression.