A review of cytogenetic studies on human zygotes and preimplantation embryos is presented. This survey documents the high incidence of chromosomal abnormalities in embryos in vitro . Up to date, 914 zygotes and embryos have been karyotyped. The rate of abnormalities is significantly higher in morphologically poor-quality embryos than in good-quality embryos (86.6 and 36.6% respectively). In both groups, aneuploidy is the most frequently observed abnormality. In addition, various types of aberrations such as polyploidy, haploidy, mosaicism or fragmentation are also found. Among tripronucleated zygotes, 81.9% display chromosomal abnormalities. Data suggest that some parameters of IVF procedures might be responsible for the occurrence of some abnormalities.