AbstractThe covalent attachment of small ubiquitin‐like modifier (SUMO) proteins to specific lysine residues of target proteins, a process termed sumoylation, is a recently discovered protein modification that plays an important role in regulating many diverse cellular processes. For this reason there is significant interest in identifying new sumoylated proteins and the lysine residue(s) within these target proteins where SUMO attachment occurs. Such knowledge will allow determination of the functional consequences of sumoylation through mutation of the relevant sequences. This unit describes two different experimental approaches for ascertaining specific protein sumoylation: the first is based on immunoprecipitation of the protein of interest followed by SUMO immunoblotting. The second involves incubation of the protein (either an in vitro translation product or a purified recombinant protein) in a reconstituted in vitro sumoylation enzymatic reaction followed by visualization of sumoylated protein as a higher than normal molecular‐weight band in SDS‐PAGE.