Basic considerations of humanity, dictates of public conscience, human dignity, notions of charity, altruism, mercy, compassion… vague ideas, instinctive impulsions only or considerations reflected in the legal realm? How legal are the expressions ‘principles of humanity’, ‘laws of humanity’, ‘dictates of public conscience’? Can one legal principle capture the ideas behind these expressions? The book explores the works of publiscists as Kelsen, Perassi, Ago, Romano, Scelle, Verdross, Cançado Trindade and Robert Kolb, maintaining at the same time sight of relevant state practice and decisions of international tribunals. It endeavors to demonstrate that the principle of humanity, in its three dimentions (subject of law, human dignity and solidarity), is a general principle of international law exercising constitutional roles in public international law.