Since its discovery, nitric oxide (NO) has been observed to play an important role in the physiology of single-celled organisms as well as high-order vertebrates. In this review, we will discuss the involvement of NO in bacterial, plant and human systems. NO originates from a variety of sources, namely bacterial, plant, and mammalian nitric oxide synthases which oxidize L-arginine. Bacterial NO is involved in toxin synthesis, signaling and biofilm formation. Organisms use NO to mediate oxidative stress incurred during the innate immune response. In plants, large amounts of NO hinder plant growth, while lower concentrations regulate normal development. NO and the associated reactive oxygen species (ROS) are effective antibacterial, anti-parasitic, and antifungal agents. Though NO has therapeutic effects in the immune system, the NO response is biphasic and concentration-dependent. NO promotes tumorigenesis within a concentration range, and induces apoptosis of cancerous cells at other concentrations. The biphasic response to NO is also evident in the regulation of chemokine, interleukins, and NF-κB, which can promote or inhibit inflammation. The physiologic response to NO is concentration dependent. NO, by way of non-adrenergic non-cholinergic (NANC) nerve transmission, propagates a cascade of molecular signaling that facilitates smooth muscle cell relaxation and increased arterial inflow into the corpora, initiating an erectile response. Additional NO is released through NOS activity in the endothelium in response to cholinergic nerve activity and shear stress, which helps to maintain erection.