Compared with other mammals, humans as a species are relatively infertile. Various studies suggest that in young, unselected couples, who are trying to conceive, 20–25% of them should be successful each monthly cycle (Bonde et al. 1998; Edwards & Brody, 1995). This compares with an average of 70% in captive baboons for example (Stevens, 1997). Interestingly, the implantation rate after in vitro fertilisation (IVF) at best averages around 20% per embryo transferred (Edwards & Beard, 1999). Evidence is steadily accumulating to prove that the major cause of implantation failure in humans after both in vivo and in vitro fertilisation is the high incidence of chromosomal abnormality. Combining data from cytogenetic studies of spontaneous abortions with those obtained from preimplantation embryos suggests that meiotically derived aneuploidy occurs in 25% of conceptions, an order of magnitude higher than is found in other well studied species such as the mouse (Hassold & Jacobs, 1984; Jamieson, 1994). Additionally, interphase fluorescent in situ hybridisation (FISH) analysis of three day old human embryos has shown that up to 50% are chromosomally mosaic (Delhanty et al. 1997; Munné et al. 1998), further increasing the chance of implantation failure. This review covers the evidence accumulating from a variety of sources that poor fertility in humans has a chromosomal basis.