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This chapter describes the data on pre-implantation genetic diagnosis (PGD) for aneuploidy screening (AS), focusing in two main topics: the chromosomal status of human gametes and embryos and the clinical application of PGD-AS in assisted reproductive technology (ART) and its controversies. In oocytes, the most frequent abnormalities detected were single chromosome trisomy and monosomy. The maternal origin of aneuploidy is prevalent in abnormal human conceptuses, and reproductive maternal aging is the important factor affecting the frequency of aneuploidy. Previous history of spontaneous abortions is a risk factor of pregnancy loss even in patients with normal karyotype. New evolving technologies will be able to significantly increase the sensitivity of PGD and reduce the diagnostic errors. Biological events such as mosaicism, postzygotes aneuploides, and trisomic rescue can cause diagnostic errors. With the view that efficiency can be significantly improved, PGD-AS has to be considered as a tool to implement ART procedure.