Covalent attachment of the small ubiquitin-like modifier (SUMO) to proteins constitutes a posttranslational modification intensely studied during the last decade. From the many proteins that serve as SUMO substrates, multiple functions have been assigned to this 100-amino acid polypeptide. Among them, a salient role in transcriptional regulation, and essentially in repression, prevails. Association of histone deacetylases (HDACs) with SUMO closely ties sumoylation with transcriptional repression. However, repressive effects linked to SUMO modification are not exclusively attributable to HDAC recruitment. Recently, several reports have revealed the importance of SUMO in the function of a variety of repressor complexes. In this respect, sumoylation is usually coupled to the establishment of heterochromatic states in the DNA. In this review, we summarize these recent reports and previous results concerning SUMO-mediated transcriptional repression. The analysis of the available data uncovers the importance not only of the covalent attachment of SUMO to proteins, but also of the presence in many proteins of SUMO interacting motifs (SIMs) that mediate non-covalent association with SUMO. In light of these findings we raise key questions and discuss why SUMO adopts a prominent role in establishing transcriptional repression in context of the activity, localization and architecture of chromatin-associated repressor complexes.