The small ubiquitin-like modifier (SUMO) is covalently attached to lysine residues in target proteins and in doing so changes the properties of the modified protein. Here we examine the role of SUMO modification in transcriptional regulation. SUMO addition to components of the transcriptional apparatus does not have a common consequence as it can both activate and repress transcription. In most cases, however, SUMO modification of transcription factors leads to repression and various models to explain this, ranging from retention in nuclear bodies to recruitment of histone deacetylases are discussed.