The reproductive physiology of postpartum cows is different from that of heifers because of the combined effects of the past pregnancy and lactation. Neither lactation nor pregnancy has a major effect on postpartum fertility when calving is free from disease and lactation is moderate. Postpartum beef cows in good body condition have conception rates nearly equivalent to those of virgin heifers once their uteri are involuted and they initiate ovarian cycles. However, cows will experience infertility when nutrient requirements for maintenance and lactation exceed nutrient intake (postpartum beef cows) or when nutrients are specifically partitioned toward lactation (postpartum dairy cows). The subsequent loss of body fat that occurs in either case has effects on a variety of reproductive processes and reproduction becomes less efficient. The mechanisms that lead to abnormal reproduction in nutritionally compromised postpartum cattle have been investigated intensively. Much of the effort has focused on the nature of the signal (endocrine or otherwise) that controls pituitary secretion of LH and FSH, the response of the ovary to LH and FSH, and other ovarian effects that are independent of gonadotrophins. Reproductive studies in ruminants have tended toward studies of follicular development and this focus relates back to solving the problem of anoestrus. Less work has been done on the effects of nutrition on the early embryo, the health of which may be predetermined by factors affecting the oocyte within the preovulatory follicle. Few studies have examined the effect of nutrition on uterine function in postpartum cattle. Solutions to postpartum reproduction will probably arise from a variety of approaches that include traditional physiology as well as more modern genomic and proteomic technologies.