Sexual transmission of HIV occurs because an infected person has unprotected sex with a previously uninfected person. The majority of HIV infections are transmitted by individuals who are unaware of their infection, and most persons who are diagnosed with HIV significantly reduce or eliminate risk behaviors once they learn they have HIV. However, a minority of known-infected individuals engage in transmission risk behavior, sometimes without disclosure to their partners. Such behavior may involve a breakdown or temporary suspension of moral mechanisms, such as personal responsibility beliefs and anticipatory self-evaluative reactions to one's behavior. The present article reviews the literature on sexual transmission risk behavior within A. Bandura's (1999) theoretical framework of moral agency. The article first reviews evidence for the operation of moral agency in transmission risk behavior and HIV status disclosure. Next, suggestive evidence is presented for the operation of mechanisms of moral disengagement described by Bandura. Finally, the article reviews a small number of interventions that have been shown to be effective in reducing transmission risk behavior, through the lens of moral agency, and make recommendations for future intervention research. (PsycINFO Database Record (c) 2009 APA, all rights reserved).