The post-translational modification SUMOylation is a major regulator of protein function that plays an important role in a wide range of cellular processes. SUMOylation involves the covalent attachment of a member of the SUMO (small ubiquitin-like modifier) family of proteins to lysine residues in specific target proteins via an enzymatic cascade analogous to, but distinct from, the ubiquitination pathway. There are four SUMO paralogues and an increasing number of proteins are being identified as SUMO substrates. However, in many cases little is known about how SUMOylation of these targets is regulated. Compared with the ubiquitination pathway, relatively few components of the conjugation machinery have been described and the processes that specify individual SUMO paralogue conjugation to defined substrate proteins are an active area of research. In the present review, we briefly describe the SUMOylation pathway and present an overview of the recent findings that are beginning to identify some of the mechanisms that regulate protein SUMOylation.