Derek Parfit (1942-2017) was one of the most important and influential moral philosophers of the late 20th and early 21st centuries. This Element offers a critical introduction to his wide-ranging ethical thought, focusing especially on his two most significant works, Reasons and Persons (1984) and On What Matters (2011), and their contribution to the consequentialist moral tradition. Topics covered include: rationality and objectivity, distributive justice, self-defeating moral theories, Parfit's Triple Theory (according to which consequentialism, contractualism, and Kantian ethics ultimately converge), personal identity, and population ethics.