For over 50 years, neuroscientists have studied the multifaceted roles of the neurotransmitter dopamine in behavior, emotion, and cognition. Recent findings regarding the rostromedial tegmental nucleus (RMTg), habenula, and other closely related structures suggests that equally powerful “anti-dopamine” systems exist in the brain whose functions are still being elucidated. Emerging evidence suggests that the RMTg and its efferents may strongly modulate dopamine systems during aversive learning, motor inhibition, and a wide range of other cognitive and behavioral processes. Many of its functions appear opposite to those of dopamine, with possible implications for understanding disorders such as depression, addiction, mania, and bipolar disorder.