Authors have defined moral injury as the “lasting psychological, biological, spiritual, behavioral and social impact” of exposure to acts that transgress deeply held moral beliefs and expectations. The idea that service members (SMs) and veterans can suffer in a lasting way psychically by events that transgress deeply held moral beliefs has captured the attention of veterans and SMs and the larger public, as well as clinicians and researchers who work with these populations. However, there are major unanswered questions about the construct validity of moral injury. For example, there is no paradigmatic way of defining moral injury as an outcome, and the parameters of what should be considered potentially morally injurious events are not clear. It is also not clear whether moral injury has incremental explanatory or clinical validity beyond posttraumatic stress disorder. This chapter will review empirical evidence about potentially morally injurious events, the moral injury syndrome, and causal models of moral injury; examine the potential similarities and differences between moral injury and PTSD; and conclude by considering issues in the field and avenues for research and construct development.