Nitric oxide (NO), a reactive nitrogen species, is a molecule of high physiological as well as pathological importance. Physiological mechanisms mediated by NO mainly include angiogenesis, growth, puberty and senescence. NO has vital roles in normal reproduction, including steroidogenesis, gametogenesis and the regulation of germ-cell apoptosis. In females, NO stimulates an inflammatory cascade to induce ovulation, decreases steroidogenesis in luteal and granulosa cells, and acts as a paracrine factor to mediate reproductive cycles and implantation. In males, NO is a key player for steroidogenesis, erectile functions, sperm capacitation and acrosome reaction. Moreover, NO is also a regulator of Sertoli cell-germ cell interaction and maintenance of the blood-testis barrier. In pathological conditions such as infections, increased nitric oxide synthase (NOS) activities stimulate the excessive synthesis of NO which acts as a proinflammatory mediator inducing oxidative stress (OS), which is detrimental to reproductive functions in both males and females. During impregnation, the overproduction of NO results in uterine epithelial cell inflammation and immune rejection of implantation. Excessive NO synthesis disrupts gonadal functions, and induces germ cell apoptosis and oxidative damage to the germ cells. This review elucidates how the differences in NO expression levels account for its beneficial and adverse impacts upon male and female fertility.