Groundwork of the Metaphysics of Morals (1785) is the first of Immanuel Kant's mature works on moral philosophy and remains one of the most important in the field.

Kant examines the core concepts and principles of moral theory, showing that they are normative for rational agents. He aspires to lay bare the fundamental principle of morality and prove that it applies to us. Kant offers the argument that the rightness of an action is determined by the character of the principle that a person chooses to act upon, in contrast to the moral sense and teleological moral theories that dominated moral philosophy at the time he was writing.

Groundwork is divided into a preface, followed by three sections. Kant's argument works from common reason up to the supreme unconditional law, in order to identify its existence. He then works backwards from there to prove the relevance and weight of the moral law. The book is famously obscure, and it is partly because of this that Kant later, in 1788, decided to publish the Critique of Practical Reason.