- The development of replacement heifers is a major economic investment for all beef and dairy operations. The costs associated with heifer development cannot be recovered if heifers do not conceive and remain productive in the herd; therefore, heifers need to conceive early in the breeding season or risk being culled. Previous research has reported up to a 21% increase in fertility from pubertal estrus to the third estrus of a heifer. The use of reproductive tract scores to determine pubertal status has demonstrated that peripubertal and pubertal heifers have increased pregnancy success to estrous synchronization compared with heifers that were prepubertal. The development of RIA has allowed accurate measurement of peripheral blood hormone concentrations associated with the pubertal process and luteal formation. This basic knowledge has increased our understanding of the mechanisms that control puberty in heifers. In addition, understanding the hormonal changes that occur during the estrous cycle has allowed for the development of estrous synchronization protocols that result in increased control of follicular growth, regression of luteal tissue, and ovulation. Transrectal ultrasonography has increased our understanding of follicular waves; this understanding led to research investigating the endocrine regulation of follicular waves and development of methods to synchronize follicular waves for purposes of fixed-time AI. Current topics of research include the effect of antral follicle count on fertility and the effect of maternal nutrition (on the fetus in utero) on subsequent reproductive potential of a heifer (i.e., fetal programming). Advancements in genomic technologies will likely provide a powerful tool for selecting heifers at birth that will have a greater probability of being reproductively successful if managed correctly. Therefore, knowledge gained through basic research on factors that control puberty has improved and will continue to improve heifer development and fertility.