Tissue morphogenesis is a fascinating aspect of both developmental biology and regeneration of certain adult organs, and timely control of cellular differentiation is a key to these processes. During development, events interrupting cellular differentiation and leading to organ failure are embryonic lethal; likewise, perturbation of differentiation in regenerating tissues leads to dysfunction and disease. At the molecular level, cellular differentiation is orchestrated by a well-coordinated cascade of transcription factors (TFs) and chromatin remodeling complexes that drive gene expression. Altering the localization, stability, or activity of these regulatory elements can affect the sequential organization of the gene expression program and result in failed or abnormal tissue development. An accumulating body of evidence shows that the sumoylation system is a critical modulator of these regulatory cascades. For example, inhibition of the sumoylation system during embryogenesis causes lethality and/or severe abnormalities from invertebrates to mammals. Mechanistically, it is now known that many of the TFs and components of chromatin remodeling complexes that are critical for development and differentiation are targets for SUMO modification, though the specific functional consequences of the modifications remain uncharacterized in many cases. This chapter will address several of the models systems that have been examined for the role of sumoylation in differentiation and development. Understanding the profound regulatory role of SUMO in different tissues should lead not only to a better understanding of developmental biology, stem cell linage control, and the mechanisms of cellular differentiation, but may also lead to the identification of new targets for drug therapy and/or therapeutic manipulation of damaged organs and tissues.