We study the mechanisms that ensure that chromosomes segregate correctly during cell division, particularly in meiosis. During this specialized cell division, diploid cells give rise to haploid gametes, such as sperm and eggs, so that diploidy is restored by fertilization. Defects in meiosis can generate gametes, and therefore embryos, with an incorrect number of chromosomes. These aberrations in chromosome number, also referred to as aneuploidy, typically produce inviable embryos. Indeed, it is estimated that 30% of human miscarriages are due to aneuploidy. In some cases, the presence of an extra copy of a chromosome can be tolerated by a human embryo but results in serious developmental disorders, such as Down and Klinefelter’s syndrome. Therefore, our research has implications for understanding and trying to prevent human infertility and birth defects.