This review assesses some scientific and ethical problems with human in vitro fertilization. Improved selection of viable embryos, better culture conditions, and greater understanding of the uterine environment will increase success and prevent multiple pregnancy. Further advances will also improve oocyte cryopreservation, in vitro maturation of oocytes, knowledge of sperm function, and sperm microinjection. Preimplantation diagnosis will help avoid genetic diseases and increase understanding of embryonic defects and the viability of zygotes. The greatest ethical problem with all these developments seems to be delivery of these complex treatments when health-care resources are increasingly limited.