Abstract: Infectious diseases, such as hepatitis A, B and C and HIV, are an important health consequence of drug addiction, especially in injecting drug users (IDUs). Hepatitis C, the most common infectious disease in IDUs, is the central focus of this monograph. Injecting drug use, through sharing injection equipment, is currently the major risk factor for hepatitis C. The great majority of new hepatitis C cases are related to injecting drug use and the prevalence and incidence of hepatitis C in IDUs are extremely high. This monograph pulls together state-of-the-art knowledge in treatment, natural history and prevention. It provides the first European data on epidemiological trends in injecting drug users, presents modelling results that give early indications to which interventions could be the most effective at the population level, as well as estimates of the future costs of healthcare, what part of these costs are avoidable, and the cost-effectiveness of existing interventions such as needle and syringe exchange and methadone treatment.