Oogenesis involves the production of an oocyte that can undergo fertilization and support early development. The stimulus that initiates embryogenesis is an increase in the concentration of intracellular Ca2+ in the cytoplasm of the oocyte at the time of fertilization. The development of the ability of the oocyte to release Ca2+ in response to the fertilizing spermatozoon is an essential step in the process of oogenesis. Mammalian oocytes are particularly useful for studying the development of Ca2+ signalling systems, owing to the series of Ca2+ oscillations generated at fertilization, compared with the monotonic Ca2+ increase seen in nonmammalian species. Recent evidence has revealed that Ca2+ release mechanisms are modified during oogenesis. The maximal sensitivity of Ca2+ release is reached in the final stages of oocyte maturation, just before the optimal time for fertilization. In this review, we consider the mechanism underlying Ca2+ release in mammalian oocytes and discuss how the release mechanisms are modified during oocyte maturation. The tight co-ordination of the differentiation of the Ca2+ signalling system with the development of the oocyte provides a means of ensuring successful activation at the time of fertilization. Finally, we consider the consequences for embryo development in circumstances in which the co-ordination is lost.