Natural moral law stands at the center of Western ethics and jurisprudence and plays a leading role in interreligious dialogue. Although the greatest source of the classical natural law tradition is Thomas Aquinas' Treatise on Law, the Treatise is notoriously difficult, especially for nonspecialists. J. Budziszewski has made this formidable work luminous. This book - the first classically styled, line-by-line commentary on the Treatise in centuries - reaches out to philosophers, theologians, social scientists, students, and general readers alike. Budziszewski shows how the Treatise facilitates a dialogue between author and reader. Explaining and expanding upon the text in light of modern philosophical developments, he expounds this work of the great thinker not by diminishing his reasoning, but by amplifying it.