Since its original publication in 1859, On Liberty has profoundly influenced philosophical and political debates about individual freedom, social authority, and progress; it remains a foundational text of the liberal tradition shaping modern democracies. This new edition advances our understanding of On Liberty in two related ways. First, it authoritatively establishes Harriet Taylor Mill as a co-author of On Liberty. Second, it supplements the text of On Liberty with illuminating passages from related essays, chapters, unpublished writings, and letters by both John Stuart Mill and Harriet Taylor Mill.