This chapter demonstrates that pre-implantation genetic diagnosis (PGD) may soon become a genuine addition to assisted reproduction in the effort of choosing the embryos with improved potential to result in viable pregnancy. The usefulness of PGD in assisted reproduction is obvious from the data on the prevalence of chromosomal abnormalities in oocytes and embryos. PGD is still a highly sophisticated procedure, involving biopsy of the oocyte and/or embryo, which may have detrimental effect on embryo development if not performed to the highest standard. In the absence of sufficient data from well-designed randomized controlled studies, the beneficial impact of PGD has also been demonstrated by the comparison of reproductive outcome in the same patients with and without PGD, with the assumption that the previous reproductive experience of the patients may serve as an appropriate control for PGD impact.