Luteinizing hormone (LH) is secreted by the pituitary gland as a heterodimeric glycoprotein acting on the gonads, regulating development and reproduction. In human of fertile age, it plays a central role in follicle development and spermatogenesis stimulating the production of steroid hormones and mediating proliferative signals. LH acts on a G-protein coupled receptor (LHCGR), shared together with the pregnancy hormone choriogonadotropin (hCG), which features specific intracellular signaling and physiological function. In this chapter, the role exerted by LH during fetal life and fertile age of humans is described.