The scientific and medical advances culminating in the introduction of in vitro fertilization of human oocytes into clinical practice are reviewed. Current methods that use clomiphene, human menopausal gonadotrophin, and both as follicular stimulants, and the endogenous LH surge or an injection of human chorionic gonadotrophin to induce ovulation are described. The effects of multifolliculation, the diurnal rhythm of the LH surge, and the collection of oocytes from the ovary are related to current clinical practice. The success of in vitro fertilization for infertile men and women is considered in relation to the nature of embryonic growth in vitro. Investigations into blastulation, hatching from the zona pellucida and the use of DNA probes for typing embryos are described. The implantation of embryos is the major remaining problem, and physiological and statistical analyses of implantation are given, comparing results from different clinics. The possibility of embryo `helping' and factors leading to multipregnancy are considered, and details are given of the incidence of abortion and the birth of children. The use of immature oocytes, the frozen-storage of embryos and methods of raising the chance of implantation are described briefly.