This article describes a study that is both multidisciplinary (combining epidemiological surveillance and socio-anthropological research) and methodological (integrating quantitative and qualitative techniques), designed to address a public health question regarding the determinants of high-risk behaviors among people who use drugs (PWUD) in relation to the transmission of HIV and HCV. Its purpose is to provide a pragmatic account for professionals working in this field of the practical challenges of this type of collaboration and to understand the mutual contributions of these two disciplines through the example of the ANRSCoquelicot survey (National Agency for AIDS Research) conducted at the Institute for Public Health Surveillance (InVS) within the Department of Infectious Diseases in collaboration with the Center for Psychotropic, mental health, and society (Cesames) and the National Institute for Demographic Studies (Ined). This article focuses primarily on the contribution of the social sciences to epidemiology, as the InVS’s approach here was to draw on socio-anthropology to complement an epidemiological approach. Beyond disciplinary considerations, this article describes the implementation of a survey in complex settings, given that the practices under study are illegal and stigmatized (drug addiction and risky behaviors). (Introduction)