Deontology is a theory about how we should act, morally speaking. It comes in several varieties, but all share certain doctrines, many of which are close to those found in the so-called 'common-sense morality' of the Western world. And all varieties are united in their opposition to consequentialism, a theory that, in its simplest form, tells us that we should always act so as to maximize impersonal value by bringing about the best consequences. This Element presents some of the different versions of deontology, including the views of W. D. Ross, and, to a lesser extent, Immanuel Kant. It defends certain deontological tenets, while challenging others, and contrasts them with consequentialism. Deontology and consequentialism are two of the main contenders in ethical theory, but virtue ethics is another, and it too is addressed (briefly), with an attempt to see it, in its most plausible form, as part of deontology.