In rodents, an entrainable circadian clock begins oscillating prenatally in the hypothalamic suprachiasmatic nuclei (SCN). The maternal circadian system coordinates (entrains) the timing of the developing clock to the prevailing light-dark cycle during both late fetal and early neonatal life. This maternal communication of circadian phase ensures that the developing animal is coordinated to the outside world until maturation of the normal pathway of light-dark entrainment in adults, the retinohypothalamic pathway, permits direct photic entrainment through the neonatal eye. The mechanism of maternal communication of circadian phase remains to be determined, but the necessity of the maternal SCN in this communication has been demonstrated. Indirect lines of evidence suggest that a similar scheme occurs for the development of circadian rhythms in humans.