Heroin use, particularly injection use, is a problem of great public health importance. The risks associated with heroin dependence, such as HIV and viral hepatitis, and the social costs due to associated crime and poverty exceed those of most other drugs of abuse. Increasing purity and decreasing cost of heroin likely contribute to trends of decreasing age at first use and an increasing rate of initiation into regular use in the United States. Effective treatment is available for heroin dependence, so primary care providers should screen patients for this disorder. This article reviews the epidemiology of heroin use and dependence in the United States and outlines what is known regarding risk factors for initiation of heroin use and for heroin dependence.