The contribution of timing cues from the environment to the coordination of early developmental processes is poorly understood. The day–night cycle represents one of the most important, regular environmental changes that animals are exposed to. A key adaptation that allows animals to anticipate daily environmental changes is the circadian clock. In this review, we aim to address when a light-regulated circadian clock first emerges during development and what its functions are at this early stage. In particular, do circadian clocks regulate early developmental processes? We will focus on results obtained with Drosophila and vertebrates, where both circadian clock and developmental control mechanisms have been intensively studied. Developmental Dynamics 236:142–155, 2007. © 2006 Wiley-Liss, Inc.