Several culture medium formulations are now available for the successful production and propagation of viable human embryos. In the most popular format, nutrients are provided in a temporal sequence that matches metabolic and amino acid composition with the requirements of specific developmental stage. An alternative philosophy, that all nutritional requirements for preimplantation embryogenesis can be met with a single medium formulation, is represented in commercially available formulations as well. Regardless of format employed, it is not widely appreciated in assisted reproductive technology laboratories that medium performance is strongly influenced by other components of the culture system, such as transitional conditions employed during the retrieval, pH, gas phase, and patient-specific characteristics. There is now further concern that in vitro culture, in general, modifies normal embryonic epigenetic processes and gene expression, genetic changes that may relate to specific ingredients of culture media.