The mental health impact of Chernobyl is regarded by many experts as the largest public health problem unleashed by the accident to date. This paper reviews findings reported during the 20-y period after the accident regarding stress-related symptoms, effects on the developing brain, and cognitive and psychological impairments among highly exposed cleanup workers. With respect to stress-related symptoms, the rates of depressive, anxiety (especially post-traumatic stress symptoms), and medically unexplained physical symptoms are two to four times higher in Chernobyl-exposed populations compared to controls, although rates of diagnosable psychiatric disorders do not appear to be elevated. The symptom elevations were found as late as 11 y after the accident. Severity of symptomatology is significantly related to risk perceptions and being diagnosed with a Chernobyl-related health problem. In general, the morbidity patterns are consistent with the psychological impairments documented after other toxic events, such as the atomic bombings of Hiroshima and Nagasaki, the Three Mile Island accident, and Bhopal. With respect to the developing brain of exposed children who were in utero or very young when the accident occurred, the World Health Organization as well as American and Israeli researchers have found no significant associations of radiation exposure with cognitive impairments. Cognitive impairments in highly exposed cleanup workers have been reported by Ukrainian researchers, but these findings have not been independently confirmed. A seminal study found a significant excess death rate from suicide in cleanup workers, suggesting a sizable emotional toll. Given the magnitude and persistence of the adverse mental health effects on the general population, long-term educational and psychosocial interventions should be initiated that target primary care physicians, local researchers, and high risk populations, including participants in ongoing cohort studies.