The toxic effects of oxygen on the embryos of various animal species are reviewed. Methodologies for assessing embryonic damage are discussed and possible ways of preventing the damage are explored. Three methods of potentially minimizing oxidative damage to human embryos were tested using gametes, zygotes, and embryos from a clinical IVF programme: (i) decreasing the oxygen tension in the gas phase used for culture during insemination, fertilization, and embryo growth; (ii) changing the formulation of culture media to include some components designed to protect against oxidative damage; and (iii) reducing the duration of insemination to minimize the effect of oxidative damage caused by spermatozoal metabolism. Fertilization, cleavage, embryo utilization, pregnancy, and embryo implantation rates were used to monitor these changes. Although all three methods gave an increase in success rates, there was still a dramatic decrease in success with patient age. It is suggested that, although the system of handling and culturing embryos can be optimized with respect to embryonic mitochondrial function, there are inherent age-related defects in oocytes and embryos that are still more fundamental than the environmental conditions of the embryo.