Over the past decade there has been a resurgence of interest in the culture media used in clinical in vitro fertilization. Unfortunately, during this time more confusion than consensus appears to have developed regarding the composition of these media. In order to facilitate a clearer understanding of this field, it is important to understand the role of specific medium components and how their use is regulated by the embryo. The roles of the key nutrients glucose, pyruvate, lactate, and amino acids during the preimplantation period have therefore been presented. Analysis of how the embryo regulates the utilization of such nutrients has led to a clearer understanding of the embryo's requirements during the dynamic period of preimplantation development. From such information, sequential culture media have been developed along with novel noninvasive tests of embryonic viability. It is proposed that continued studies on the human embryo will lead to further improvements in embryo culture conditions and the optimization of viability assays, culminating in the ability to transfer single embryos for the majority of, if not all patients.