Pre-implantation genetic diagnosis (PGD) is generally defined as the testing of pre-implantation stage embryos or oocytes for genetic defects. It has been developed for couples whose potential offspring are at risk of severe Mendelian disorders, structural chromosome abnormalities or mitochondrial disorders. Pre-implantation embryo diagnosis requires in vitro fertilization, embryo biopsy and either using fluorescent in situ hybridization or polymerase chain reaction at the single cell level. Therefore, it is a complex procedure which requires much experience. Aneuploidy screening to improve medically assisted reproduction (in vitro fertilization/intracytoplasmic sperm injection) is a variant type of PGD. The past, present and future of this development are strongly related to the natural occurrence of chromosomal mosaicism in the pre-implantation embryo. PGD should be included in each reproductive health care programme. It is recognized as an important alternative to pre-natal diagnosis. However, diagnosis from a single cell remains a technically challenging procedure, and the risk of misdiagnosis cannot be eliminated. An ethical discussion of the question of whether PGD is acceptable at all–the ‘desirability question’–is a rearguard action. Discussion must primarily focus on the conditions of exercising due caution in and the dynamics of PGD.