The social teaching of the Church gives a special importance to inequalities within society, especially in connection with the main principles of human dignity and justice. A morally powerful source of social inequality is systematic evil. The concept under different names (structures of sin, social sin, structural sin) was especially prominent in the social teaching of Pope John Paul II, culminating in his encyclical letter Sollicitudo Rei Socialis; but used even before that, in his apostolic exhortation Reconciliatio et Pænitentia. The concept has finally found its way into the Catechism of the Catholic Church (1992), where it is defined as follows: "Structures of sin are social situations or institutions that are contrary to the divine law. They are the expression and effect of personal sins." After looking at the history of the concept, the paper examines the relationship between personal sin and structural sin (institutional sin), especially in connection with personal as well as social guilt and responsibility. Within moral theology, hamartiology studies the reality of sin. The paper is interested in the way in which, apart from the well-studied personal sin, evil is present and active in the world through social structures, and in what kinds of moral-theological problems this creates. The experiences of the twentieth century with the two totalitarian dictatorships have taught us that we have to take very seriously the presence of evil in the world, not just in its personal form, but also through its structural, institutional, and systematic workings. How is this experience different from those of earlier ages? Systematic evil is a grave force present and active in the world, when it is not only personal wrong decisions and wrongdoings that corrupt the workings of the created order, but evil is also powerful in the entities of the whole human society, contributing among others to relationships of inequality in the social order.