Derek Parfit, who died in 2017, is widely believed to have been the most significant moral philosopher in well over a century. The twenty-one new essays in this book have all been inspired by his work. They address issues with which he was concerned in his writing, particularly in his seminal contribution to moral philosophy, Reasons and Persons (OUP, 1984). Rather than simply commenting on his work, these essays attempt to make further progress with issues, both moral and prudential, that Parfit believed matter to our lives: issues concerned with how we ought to live, and what we have most reason to do. Topics covered in the book include the nature of personal identity, the basis of self-interested concern about the future, the rationality of our attitudes toward time, what it is for a life to go well or badly, how to evaluate moral theories, the nature of reasons for action, the aggregation of value, how benefits and harms should be distributed among people, and what degree of sacrifice morality requires us to make for the sake of others. These include some of the most important questions of normative ethical theory, as well as fundamental questions about the metaphysics of personhood and personal identity, and the ways in which the answers to these questions bear on what it is rational and moral for us to do.