The genomes of virtually all sexually reproducing species contain transposable elements. Although active elements generally transpose more rapidly than they are inactivated by mutation or excision, their number can be kept in check by purifying selection if its effectiveness becomes disproportionately greater as their copy number increases. In sexually reproducing species, such synergistic selection can result from ectopic crossing-over or from homologous recombination under negative epistasis. In addition, there may be controls on transposon activity that are associated with meiosis. Because a sexual lineage that abandons sex must lack such mechanisms, it may be driven to extinction by the unchecked proliferation of deleterious transposons inherited from its sexual progenitor. An important component of the evolutionary advantage of sex over asex may therefore lie in the ability of sex, despite facilitating the spread of deleterious elements within interbreeding populations, also to restrain their intragenomic proliferation. BioEssays 27:76–85, 2005. © 2004 Wiley Periodicals, Inc.