The rostromedial tegmental nucleus (RMTg), a brake of the dopamine system, is specifically activated by aversive stimuli, such as foot shock. It is principally composed of gamma-aminobutyric acid neurons. However, there is no exact location of the RMTg on the brain stereotaxic atlas. The RMTg can be defined by c-Fos staining elicited by psychostimulants, the position of retrograde-labeled neurons stained by injections into the ventral tegmental area (VTA), the terminal field formed by axons from the lateral habenula, and some molecular markers identified as specifically expressed in the RMTg such as FoxP1. The RMTg receives a broad range of inputs and produces diverse outputs, which indicates that the RMTg has multiple functions. First, the RMTg plays an essential role for non-rapid eye movement sleep. Additionally, the RMTg serves a vital role in response to addiction. Opiates increase the firing rates of dopaminergic neurons in the VTA by acting on μ-opioid receptors on RMTg neurons and their terminals inside the VTA. In this review, we summarize the recent research advances on the anatomical location of the RMTg in rats and mice, its projections, and its regulation of sleep–wake behavior and addiction.