Idiosyncratic drug-induced liver injury (DILI) is a significant health problem because of its unpredictable nature, poorly understood pathogenesis, and potential to cause fatal outcomes. It is also a significant hurdle for drug development and marketing of safe prescription medications. Idiosyncratic DILI is generally rare, but its occurrence is likely underappreciated due to the lack of active reporting or surveillance systems and substantial challenges involved in its recognition and diagnosis. Nonetheless, DILI is a common cause of potentially serious and fatal acute liver failure in both children and adults. Population-based studies that accurately estimate the incidence and full spectrum of DILI are limited. However, using a prospective, population-based French study with an annual estimated incidence of 13.9 +/- 2.4 DILI cases per 100,000 inhabitants, it has been extrapolated that nearly 44,000 individuals in the United States will suffer from DILI each year. Although increasing numbers of patients are also being seen with DILI due to herbal and dietary supplements, the epidemiology of this entity requires further investigation. In this article, the epidemiology of DILI, both in the general population and in potentially high-risk subgroups, is reviewed.