The far reaching changes in man's social and personal life taking place in our lifetime underline the need for a sound ethical evaluation of our rights and duties and of human behaviour both on the individual level and in the political society. On many issues judgments of value vary widely and a consultation of the thought of Thomas Aquinas on the basic questions will be helpful, the more since he is not only one of the greatest philosophers but also succeeded in integrating in his moral philosophy the wisdom of the ancients, in particular of Aristotle and the Stoa. This book presents Aquinas's thought on such central questions as man's happiness, how to determine the morality of our actions, the natural law and the main virtues, as well as on the common good, war, human labour, love and friendship. Throughout the book the intellectual character of this moral philosophy is pointed out and problems are set in a historical perspective.