Moral injury is defined as the profound suffering that can arise from involvement in either perpetrating, failing to prevent, or witnessing actions that transgress deeply held moral norms and expectations or from betrayal by a trusted leader or authority. Originally arising from attempts to understand the experiences of traumatized veterans, the construct has expanded in relevance to an increasingly wider range of settings—particularly in the wake of the COVID-19 pandemic, which saw increased pressure and diminished resources, heightening the moral injury risk for several professionals, including frontline health workers and emergency services. Throughout this entry, we demonstrate the key situations that can lead to moral injury, as well as the most widely accepted symptomatic profile of the syndrome. We differentiate between moral injury and the highly comorbid condition of post-traumatic stress disorder. We also explain why human morality provides the blueprint by which human community and social identity are formed, which in turn provides rationale for why experiences of moral pain and moral injury are so intensely distressing, and lead to a range of functional impairments. We provide rationale for why the experience of moral injury is a universal one and one highly relevant to a broader range of settings beyond military research. We also discuss the conceptual confusion around the construct and refer to key measurement tools for moral injury in both the military research space and other occupational contexts.