This Companion offers a state-of-the-art survey of the work of John Stuart Mill -- one which covers the historical influences on Mill, his theoretical, moral and social philosophy, as well as his relation to contemporary movements. Its contributors include both senior scholars with established expertise in Mill's thought and new emerging interpreters. Each essay acts as a go-to resource for those seeking to understand an aspect of Mill's thought or to familiarise themselves with the contours of a debate within the scholarship. The Companion is a key reference on Mill's theory of liberty and utilitarianism, but also provides a valuable resource on lesser-known aspects of his work, including his epistemology, metaphysics, and philosophy of language. The volume is divided into six sections. Part I covers Mill's life, his immediate posthumous reputation, and his own telling of his life-story. Part II brings together an accessible and comprehensive summary of the various influences on Mill's thought. Part III offers an account of the foundations of Mill's philosophy and his thought on key philosophic topics. Parts IV and V tackle issues from Mill's moral and social philosophy. Part VI concludes with a treatment of the broader aspects of Mill's thought, tracing his relation to major movements in philosophy.