The seven articles in this volume are based on lectures presented at the annual symposium, Some Mathematical Questions in Biology, held in conjunction with the American Institute of Biological Sciences meeting in Toronto, Ontario in August 1989. Sexual selection, sex determination, and sex allocation have been at the center of evolutionary ecology since its inception and have played an important role in the development of many concepts. As this volume demonstrates, many key questions remain to be investigated through a combination of empirical and theoretical work. In addition, questions of sex provide a natural mechanism for crossing the great taxonomic divide by allowing plant and animal researchers to focus on similar kinds of questions using a wide variety of organisms.