Moral injury is a hypothetical construct that is commonly accepted to entail multidimensional impairment and suffering experienced after severe transgressions, committed either by oneself or others, of one’s moral values. 1 Acts that involve violations of one’s moral beliefs and expectations are called Potentially Morally Injurious Events (PMIEs) because events, no matter how egregious, are only potentially damaging. Moral transgressions and suffering as a result of those transgressions are part of the human condition; it is only recently that mental and behavioral health care communities have begun to regard perceived transgressions as potential traumas causing lasting harm. Moral injury is proposed by some to lead to a complex set of outcomes, such as unbidden memories and intrusive emotional reminders of the PMIE, avoidance and emotional numbing, embitterment and rage, guilt and shame, and maladaptive attempts to cope, such as substance misuse, social withdrawal and isolation, or, in extreme cases, suicide. The idea of moral injury as a clinical syndrome was developed by US psychologists, working with combat veterans, who recognized the inadequacy of models of trauma that emphasize fear and victimization experiences. Unfortunately, there is no field-wide consensus about which events should qualify as PMIEs, the clinical picture of moral injury, or how exactly moral injury develops after PMIEs. Because of these open questions, it is not clear whether and to what extent moral injury is distinct from posttraumatic stress disorder (PTSD). This chapter will review the development of the moral injury syndrome to explore how and why moral injury emerged as a concept and where the moral injury field stands today, ten years after its introduction