Drug injectors are at risk for infection with human immunodeficiency virus (HIV) and other blood-borne pathogens through the exchange of (infected) blood resulting from unhygienic injecting practices. Research attention and public discussion have focused primarily on the sharing of syringes and needles. While the focus on syringe sharing has sparked important interventions (bleach distribution, syringe exchange) it may have obscured the social relationship in which injecting equipment is used. Drug sharing plays a crucial role in the social organization of the drug using subculture. In this paper, various drug sharing practices and other distinguishable aspects of the injecting process-collectively termed Syringe-Mediated Drug Sharing (SMDS)-are described. All of these behaviors may put injecting drug users (IDUs) at risk for infection. The purpose of this paper is to stimulate scientific inquiry into SMDS behaviors and the social contexts which shape them. Descriptions are based primarily on field studies in Rotterdam and New York City. Recommendations for safer injecting training and education are proposed, as are directions for future research.