This chapter demonstrates that in unselected patient populations there is no benefit, whilst in women classified as poor or hyporesponders to follicle-stimulating hormone (FSH) and those of advanced reproductive age, luteinizing hormone (LH) supplementation improves assisted reproductive technology (ART) outcomes. During natural menstrual cycles, FSH levels initially rise and then, primarily under the influence of oestradiol, decline, whereas LH is secreted in pulses, rising during the mid-follicular phase of the cycle. Supplementation with LH can be provided through treatment with recombinant human LH (r-hLH) or urinary derived human menopausal gonadotropin (hMG). There have been two meta-analyses published comparing the utilization of r-hLH plus FSH vs FSH alone in women co-treated with gonadotropin-releasing hormone (GnRH) agonists and antagonists in ART protocols. In women undergoing pituitary down-regulation with GnRH agonist long protocols, LH levels can fall to below those that characterize hypogonadotrophic hypogonadism (HH).