Libido refers to a fluctuating state of sexual motivation in all organisms. Sexual motivation is altered by internal factors, such as circulating steroid hormone levels and feedback from sexual stimulation; external factors, such as the presence of sexually relevant incentives; and by the cognitive processing of these factors that provides variations in sexual arousability and expectation of sexual reward. Libido thus reflects constant fluctuations in sexual arousal, desire, reward, and inhibition. Recent advances in neurochemical detection, pharmacologic analyses, and brain imaging, have helped identify neuroanatomic and neurochemical systems that regulate these four aspects of sexual function. Another important factor is the activation of central monoamine and neuropeptide systems that link incentive motivation, reward, and inhibition together with autonomic pathways that detect and relay sexual arousal. The activation of these systems by steroid hormones, and modulation by expectancy of sexual reward, are critical features of the neural “state” in which reactivity to sexual incentives is altered.