Injecting is one of the main causes of health damage related to illegal drug use today. It is strongly related to fatal and non-fatal overdoses, as well as to the transmission of infectious diseases, such as hepatitis B and C, and HIV infection. Yet studying drug injection is no easy task, due to its hidden nature and low prevalence in general population terms. It is necessary to understand why and how people decide to engage in such dangerous behaviour, and to analyse how drug injectors perceive risk. There are fundamental issues that need to be understood in order to interpret and better use epidemiological quantitative data on the subject. This publication is the result of a review undertaken by the EMCDDA working group on injecting drug use and risk behaviour. It helps in understanding injecting drug use and its adverse health consequences, particularly risk behaviour associated with the transmission of HIV.