The reversible conjugation of the small ubiquitin-related modifier (SUMO) peptide to protein substrates (sumoylation) is emerging as a major post-translational regulatory process in animals and other eukaryotes, including plants. Database annotation, and genetic and biochemical analyses indicate that components of the SUMO conjugation and deconjugation systems are conserved in plants such as Arabidopsis, rice, tomato, and Medicago. Specifically, Arabidopsis AtSUMO1/2 and SUMO E2 conjugation enzyme AtSCE1a are implicated in abscisic acid (ABA) responses and the ubiquitin-like SUMO protease 1 (ULP1) AtESD4 in flowering time regulation. The AtSIZ1 SUMO E3 ligase functions in phosphate starvation responses, cold tolerance, basal thermotolerance, salicylic acid (SA)-dependent pathogen defense, and flowering time regulation. Following is a brief overview of the current understanding of SUMO conjugation and deconjugation determinants, and biological processes that are regulated in plants.