Preimplantation genetic diagnosis (PGD) identifies genetic abnormalities in preimplantation embryos prior to embryo transfer. PGD is an exciting technology that may improve the likelihood of a successful pregnancy and birth for five distinct patient groups:(1) those with infertility related to recurrent miscarriages or unsuccessful in vitro fertilization (IVF) cycles,(2) those with unexplained infertility,(3) advanced maternal age,(4) severe male factor infertility, and (5) couples at risk for transmitting a hereditary disease to their offspring. PGD is always performed following an IVF cycle where multiple oocytes are retrieved and fertilized. Sophisticated techniques such as multiprobe, multicolor fluorescence in situ hybridization are used to test single cells for structural or numerical chromosome abnormalities, whereas the polymerase chain reaction, linkage analysis, and DNA sequencing are used to analyze single cells for disease-specific DNA mutations. PGD allows one to transfer only those embryos identified as being free of genetic abnormalities, thus potentially increasing the implantation rate and decreasing the miscarriage rate. These technologies identify embryos free of specific genetic abnormalities and may increase the likelihood of achieving the patient's goal: the birth of a healthy infant.