A conflict of interest is a clash between an individual's professional responsibilities and their personal, typically financial, interests. Traditional economics has not shed much light on conflicts of interest, perhaps in part because it has not recognized the importance of professionalism as a motive in human behavior. In this talk I will present results from a variety of studies that examine the behavioral economics of conflict of interest. Focusing mainly on conflicts of interest in medicine, some of the research shows how people who care deeply about behaving in a professional fashion can be corrupted by economic incentives. Other research shows how disclosing conflicts of interest, far from helping the recipient of information, can backfire, helping the advice-giver and hurting the advice recipient.