A number of research and clinical studies have confirmed high frequency of cytogenetic abnormalities in human oocytes and embryos. Chromosomal imbalance in oocytes can result from loss or gain of individual chromatids or whole chromosomes. The underlying causes remain to be fully elucidated, but important modifying factors include the number and location of chiasmata and maternal age. Aneuploidy is a major cause of congenital abnormalities, mental retardation, and miscarriage. However, most of the chromosome abnormalities detected in human embryos are likely to be lethal at very early embryonic stages, and are probably incompatible with the formation of a clinical pregnancy. Many fertility clinics now screen the embryos produced during in vitro fertilization (IVF) cycle in order to identify those that are chromosomally normal. Screening the oocytes/embryos from these patients for aneuploidy using PGS may be particularly beneficial in terms of IVF outcome.