Kantian ethics refers to a deontological ethical theory developed by German philosopher Immanuel Kant that is based on the notion that "I ought never to act except in such a way that I could also will that my maxim should become a universal law." It is also associated with the idea that "it is impossible to think of anything at all in the world, or indeed even beyond it, that could be considered good without limitation except a good will." The theory was developed in the context of Enlightenment rationalism. It states that an action can only be moral if it is motivated by a sense of duty, and its maxim may be rationally willed a universal, objective law.

Central to Kant's theory of the moral law is the categorical imperative. Kant formulated the categorical imperative in various ways. His principle of universalizability requires that, for an action to be permissible, it must be possible to apply it to all people without a contradiction occurring. Kant's formulation of humanity, the second formulation of the categorical imperative, states that as an end in itself, humans are required never to treat others merely as a means to an end, but always as ends in themselves. The formulation of autonomy concludes that rational agents are bound to the moral law by their own will, while Kant's concept of the Kingdom of Ends requires that people act as if the principles of their actions establish a law for a hypothetical kingdom.

The tremendous influence of Kant's moral thought is evident both in the breadth of appropriations and criticisms it has inspired and in the many real world contexts in which it has found application.

Although all of Kant's works develop his ethical theory, it is most clearly defined in the Groundwork of the Metaphysics of Morals, the Critique of Practical Reason, and the Metaphysics of Morals. While the Groundwork of the Metaphysics of Morals is important for understanding Kant's ethics, one will get an incomplete understanding of his moral thought if one only reads the Groundwork of the Metaphysics of Morals and the Critique of Practical Reason, or is not at least aware that his other ethical writings discuss other important details about Kant's moral philosophy as a whole since "one is all the more misled if he is not aware that they form only part of the picture."

As part of the Enlightenment tradition, Kant based his ethical theory on the belief that reason should be used to determine how people ought to act. He did not attempt to prescribe specific action, but instructed that reason should be used to determine how to behave.

In his combined works, Kant construed as a basis for the ethical law by the concept of duty. Kant began his ethical theory by arguing that the only virtue that can be an unqualified good is a good will. No other virtue, or thing in the broadest sense of the term, has this status because every other virtue, every other thing, can be used to achieve immoral ends. For example, the virtue of loyalty is not good if one is loyal to an evil person. The good will is singularly unique in that it is always good and maintains its moral value regardless of whether or not it achieves its moral intentions. Kant regarded the good will as a single moral principle that freely chooses to use the other virtues for genuinely moral ends.

For Kant, a good will has a broader conception than a will that acts from duty. A will that acts from duty alone is distinguishable as a will that overcomes hindrances in order to keep the moral law. A dutiful will is thus a special case of a good will that becomes visible in adverse conditions. Kant argues that only such acts performed with regard to duty have moral worth. This is not to say that acts performed merely in accordance with duty are worthless (these still merit approval and encouragement), but that distinctively moral esteem is given to acts that are performed out of duty, or from duty, alone.

Kant's conception of duty does not entail that people perform their duties grudgingly. Although duty often constrains people and prompts them to act against their inclinations, it still comes from an agent's volition: they desire to keep the moral law from respect of the moral law. Thus, when an agent performs an action from duty it is because their moral incentives are chosen over and above any opposing inclinations. Kant wished to move beyond the conception of morality as externally imposed duties, and present an ethics of autonomy, when rational agents freely recognize the claims reason makes upon them.