In this paper a hypothesis is presented which relates chromosome pairing and sterility in males. This hypothesis has been formulated on the basis of data from numerous meiotic systems in the male of <i>Drosophila melanogaster, </i>where the sex chromosomes have heterochromatic pairing sites, sites which must interact in order for postmeiotic development to be normal. The predictions of this theory have been tested in three principal situations: (1) in various sex chromosome systems of man, the mouse, voles, and beetles; (2) in the B chromosome system of the grasshopper <i>Myrmeleotettix maculatus; </i>and (3) in univalent autosomal cases in man and in translocation heterozygotes of the mouse. In all three situations a striking correlation has been obtained between reduction or absence of pairing, on the one hand, and gametogenic breakdown, on the other. It is concluded that a saturation of pairing sites between homologous chromosomes, whether sex chromosomes or autosomes, is essential for regular meiotic or postmeiotic development.