Some methodological details regarding a survey conducted in 2008–2009 by the French Monitoring Center for Drugs and Drug Addiction (OFDT) and the SAMU Social Monitoring Center, in partnership with the National Institute for Health Prevention and Education (INPES), among 1,954 homeless individuals housed in Maternal Centers, social reintegration centers (CHRS), and emergency shelters (CHU). The objective was to explore the specific characteristics in terms of substance use and health behaviors of a population largely overlooked by traditional quantitative survey methods. This study confirms the value of conducting research among vulnerable populations using standardized measurement tools.