Although the modification of cellular factors by SUMO is an essential process in Saccharomyces cerevisiae, the identities of the substrates remain largely unknown. Using a mass spectrometry-based approach, we have identified 271 new SUMO targets. These substrates play roles in a diverse set of biological processes and greatly expand the scope of SUMO regulation in eukaryotic cells. Transcription appears to be the most prevalent process associated with sumoylation with novel SUMO substrates found in basal transcription machinery for RNA polymerases I, II, and III, pol II transcriptional elongation complexes, and a variety of chromatin remodeling, chromatin modifying, and chromatin silencing complexes. Additionally, our global analysis has revealed a number of interesting biological patterns in the list of SUMO targets including a clustering of sumoylation targets within macromolecular complexes.