Although haploidy has not been observed in vertebrates, its natural occurrence in various eukaryotic species that had diverged from diploid ancestors suggests that there is an innate capacity for an organism to regain haploidy and that haploidy may confer evolutionary benefits. Haploid embryonic stem cells have been experimentally generated from mouse, rat, monkey, and humans. Haploidy results in major differences in cell size and gene expression levels while also affecting parental imprinting, X chromosome inactivation, and mitochondrial metabolism genes. We discuss here haploidy in evolution and the barriers to haploidy, in particular in the human context. Naturally occurring haploidy in eukaryotic species (albeit not in vertebrates) with diploid ancestors suggests evolutionary benefits for the state and an innate capacity for organisms to regain haploidy. Sagi and Benvenisty discuss haploidy in evolution and haploidy barriers in light of recent isolation of haploid embryonic stem cells.