Studies of human cleavage stage embryos, 3 days after fertilization of the oocyte, have revealed remarkably high levels of chromosome abnormality. In addition to meiotic errors derived from the gametes, principally the oocyte, mitotic errors occurring after fertilization are also common, leading to widespread chromosomal mosaicism. The prevalence of chromosome anomalies in embryos may explain the relatively poor fertility and fecundity in humans and the low success rates of assisted reproductive treatments (e.g., IVF). While much is known concerning the incidence of aneuploidy during the first 3 days following fertilization, it is only in the last couple of years that large numbers of embryos at the final stage of preimplantation development, the blastocyst stage, 5 days after fertilization, have been subjected to detailed analysis. Here we discuss the latest data from the comprehensive cytogenetic analysis of blastocysts. These findings indicate that the majority of selection against chromosome abnormalities does not occur until the time of implantation or shortly after, with aneuploidy typically affecting more than 50% of blastocysts. Additionally, clinical results presented suggest that screening of blastocyst stage embryos for chromosome abnormality, with preferential transfer to the uterus of those found to be euploid, may help to improve the success rates of assisted reproductive treatments.