Why should we be held responsible for the harm we cause? This work proposes a theory of responsibility according to which, to be responsible, it is sufficient to have intervened in the world. To act and to be responsible is to assume certain risks, so that responsibility can be a matter of luck rather than fault or merit. Whether responsibility carries with it moral blame or legal liability is an important but secondary question. With the help of this theory, Honore explains the moral basis of strict liability and of tort law in general; shows when there is a moral difference between positive acts and omissions; and indicates the extent to which the circumstances that cause a wrongdoer to do wrong should affect his responsibility.