pioid addiction is one of the most common substance-related disorders worldwide. It is responsible for the majority of the morbidity and mortality caused by drugs in the population (1). Opioids include both synthetic (e.g. heroin, methadone, buprenorphine, fentanyl) and plant-derived substances (opiates, e.g. codeine and morphine). Opioids carry major risks of physical and pharmacological dependency (2). Intravenous use, in particular, is associated with a nonnegligible risk of communicable diseases (3) or death due to overdose or the long-term consequences of use (4). Finally, there is an increased risk of criminal behavior, specifically drug-related crime (5). Knowing how many individuals are addicted to opioids is important for setting health policy (6). In the first instance, calculations in Germany concern addiction caused by taking illegal opioid-containing substances. A preliminary national estimate for Germany as a whole in 1989, based on treatment data, gave a figure of 60 000 to 80 000 individuals who were problem users of opiates, cocaine, stimulants, or hallucinogenic drugs (7). A German expert group estimated the number of heroin users in western and eastern Germany in 1995 at 127 000 to 152 000 (8); for the same year, the number of intravenous drug users in western Germany and Berlin was estimated at a mean of 150 000 (97 000 to 204 000) on the basis of a survey among general practitioners (9). As part of estimating the number of individuals with problematic drug use in European Union countries, figures of 127 000 to 190 000 opiate users in Germany for the year 2000 were found using various methods. This calculation was based on treatment, police, and mortality data (10). Using these approaches, comparative estimates for 1990, 1995, and 2000 indicated a moderate increase in the number of opiate users (11). The aim of this study was to estimate the number of individuals addicted to opioids in Germany and its individual federal states for the calendar year 2016. Method This estimate is based on substitution treatment registry data, data from inpatient and outpatient addiction care statistics, and counts in 5 low-threshold addiction care Summary Background: Opioid addiction is one of the most common substance-related disorders worldwide, and morbidity and mortality due to opioid addiction place a heavy burden on society. Knowing the size of the population that is addicted to opioids is a prerequisite for the development and implementation of appropriate health-policy measures. Methods: Our estimate for Germany for 2016 is based on an enumeration of opioidaddicted persons who were entered in a registry of persons receiving substitution therapy, an enumeration of persons receiving outpatient and inpatient care for addiction without substitution therapy, an extrapolation to all addiction care facilities, and an estimation of the number of opioid-addicted persons who were not accounted for either in the substitution registry or in addiction care. Results: The overall estimate of the number of opioid-addict...