Over the past decade, a serine–threonine kinase first characterized for its role in glycogen metabolism has shown itself to be a key player in numerous processes, in organisms ranging from yeast to mammals (reviewed in Yost et al., 1997). Glycogen synthase kinase-3 was originally identified as a constitutively active kinase that is inactivated in response to extracellular signals. However, it is now clear that extracellular signals can also activate GSK-3 to direct developmental patterning and that GSK-3’s function is mediated both by phosphorylation and by its interaction with activating and inhibitory binding partners. In this review we focus on three organisms that highlight the varied roles that GSK-3 plays in embryonic development, Xenopus, sea urchin, and Dictyostelium, and discuss the molecular mechanisms that regulate GSK-3 activity in each species.