Pivotal in HIV/STD intervention and prevention is how individuals perceive risk. This article discusses how ethnotheory can be used to understand the underpinnings of risk perception from the subject's perspective. Ethnotheory's theoretical and methodological character extends present models used to explain sex-related HIV behaviors and provides insights into risk perception useful for the development of effective intervention strategies. Four detailed ethnographic excerpts from low-income African American and Mexican American men and women illustrate how people form perceptions of risk and how information gained through ethnotheory is useful in the design of culturally relevant interventions.