Research on ethical dilemmas in health care has become increasingly salient during the last two decades resulting in confusion about the concept of moral distress. The aim of the present paper is to provide an overview and a comparative analysis of the theoretical understandings of moral distress and related concepts. The focus is on five concepts: moral distress, moral stress, stress of conscience, moral sensitivity and ethical climate. It is suggested that moral distress connects mainly to a psychological perspective; stress of conscience more to a theological-philosophical standpoint; and moral stress mostly to a physiological perspective. Further analysis indicates that these thoughts can be linked to the concepts of moral sensitivity and ethical climate through a relationship to moral agency. Moral agency comprises a moral awareness of moral problems and moral responsibility for others. It is suggested that moral distress may serve as a positive catalyst in exercising moral agency. An interdisciplinary approach in research and practice broadens our understanding of moral distress and its impact on health care personnel and patient care.