In developmental biology, localization is everything. The same stimulus—cell signaling
event or expression of a gene—can have dramatically different effects depending on the
time, spatial position, and cell types in which it is applied. Yet the field has long lacked the
ability to deliver localized perturbations with high specificity in vivo. The advent of
optogenetic tools, capable of delivering highly localized stimuli, is thus poised to profoundly
expand our understanding of development. We describe the current state-of-the-art in
cellular optogenetic tools, review the first wave of major studies showcasing their application
in vivo, and discuss major obstacles that must be overcome if the promise of developmental
optogenetics is to be fully realized.