Traditionally, discussions in medical ethics have focused on ways in which medical professionals may do wrong by directly harming or wronging patients or research subjects. However, medical professionals may also act wrongly in indirect ways, by being accomplices to others' wrongdoing (e.g. by taking part in torture, a doctor may become complicit in torture). Moreover, the effects of medical complicity can aggregate to produce substantial wrongs, such as gross human right violations (e.g. the complicity of doctors in Nazi eugenics facilitated genocide). But medical complicity is also ubiquitous in everyday medicine (e.g. nurses assisting in wrongful end-of-life practices may become complicit in these practices). Professional guidelines frequently give no clear guidance on how to deal with medical complicity. They either do not address the issue, or give conflicting advice (e.g. the same codes that condemn complicity in torture also state that the physician should act in the best interest of the patient, but these professional obligations could conflict when the patient is a torture victim).Though medical complicity can have significant effects and is presenting medical professionals with unresolved ethical dilemmas, problems of medical complicity have received little sustained theoretical treatment in medical ethics. Most existing discussions either remain at the level of complex philosophical theory, or at the level of practical decision-making with little philosophical basis. This project aims to bridge this gap. It will do so by developing a clear account of medical complicity, and by deploying this account to yield concrete guidance for medical professionals and policymakers confronted with dilemmas of medical complicity. To achieve these aims, I will draw on novel philosophical analysis, relevant empirical findings, and my own earlier study of complicity in the context of embryonic stem cell research.