This paper describes some of the efforts of an interdisciplinary research team investigating the transmission of human immunodeficiency virus (HIV), the causative pathogen associated with the acquired immunodeficiency syndrome (AIDS) and related conditions. The risk groups studied were injecting drug users and their sexual partners. Due to the clandestine nature of illicit drug use, we were faced with two interrelated problems: developing a scientific method to monitor the spread of the HIV infection among these drug users and their sexual partners, groups generally thought to be especially difficult to reach; and creating a health education intervention that would help stop the epidemic from spreading among this population and through them to other members of the community. The method we developed to sample injecting drug users is called targeted sampling. Although it incorporates some aspects of other well established sampling strategies, it is sufficiently different to be treated as a separate research method. Further, targeted sampling provides a cohesive set of research methods that can help researchers study health or social problems that exist among populations that are difficult to reach because of their attributed social stigma, legal status, and consequent lack of visibility.