Preimplantation genetic testing (PGT), until recently known as preimplantation genetic diagnosis (PGD), is an early form of prenatal testing for couples at high risk of transmitting a genetic condition to their offspring, either for monogenic disorders (PGT-M) or chromosomal structural rearrangements (PGT-SR). The goal is to test for the specific genetic status in cells biopsied from oocytes/zygotes or embryos obtained in vitro through assisted reproductive technology (ART) and, following analysis, to transfer to the uterus only those embryos identified as genetically suitable relative to the condition under consideration. The selective transfer of unaffected embryos to the uterus for implantation means that PGT minimizes the need to consider the termination of affected pregnancies. This advantage of PGT means that it has become a widely acceptable alternative to conventional prenatal diagnosis. Of course, PGT is not applied without some ethical concerns. In patients with high risk for transmitting specific genetic conditions to their offspring, including serious single gene disorders and chromosomal rearrangements, there is usually little ethical debate. However, the increased availability and emergence of new uses, such as PGT for autosomal dominant late-onset disorders, cancer predisposition syndromes, PGT for histocompatibility (HLA) typing, and mitochondrial DNA (mtDNA) mutations, are associated with greater ethical controversy and important ethical questions (see Chapter 15).