Until recently, fertility research has focused almost exclusively on the role of the female partner in reproduction. This chapter discusses the one last proximate determinant of natural fertility, namely, the age at marriage. It explores whether any important component of fertility variation can be ascribed to factors operating solely in the male partner. In contrast to research on female reproductive senescence, work on age-related physiological changes in the male has yielded surprisingly few consistent and unambiguous findings. Contradictory results have emerged from studies of testosterone secretion, Ley dig cell maintenance, hypothalamo- pituitary function, spermatogenesis, and the quality of sperm. Declining testosterone production appears to be the most consistent feature of male reproductive senescence, although its causes remain uncertain. Declining hypothalamo-pituitary function may also occur, although it is far from established. The age-related reduction in testosterone in turn seems to lead to reduced spermatogenesis or impaired sperm motility and possibly a reduction in male libido.