SUMO, a small ubiquitin-like modifier, is a highly conserved post translational modification and a central regulatory system in eukaryotes. Sumoylation modulates the activities of multiple proteins, mainly in the nucleus, such as transcription factors, chromatin modifiers, and proteins involved in DNA replication and repair. However, SUMO also modifies substrates in the cytoplasm, mitochondria, plasma and ER membrane. This review summarizes our current knowledge on the functions of sumoylation in C. elegans development. SUMO modification is highly reversible and several examples described here establish its function as a molecular switch during embryogenesis and postembryonic organogenesis.