Derek Parfit, who died in 2017, is widely believed to have been the best moral philosopher in well over a century. The twenty new essays in this book were written in his honour and have all been inspired by his work—in particular, his work in an area of moral philosophy known as 'population ethics', which is concerned with moral issues raised by causing people to exist. Until Parfit began writing about these issues in the 1970s, there was almost no discussion of them in the entire history of philosophy. But his monumental book Reasons and Persons (OUP, 1984) revealed that population ethics abounds in deep and intractable problems and paradoxes that not only challenge all the major moral theories but also threaten to undermine many important common-sense moral beliefs. It is no exaggeration to say that there is a broad range of practical moral issues that cannot be adequately understood until fundamental problems in population ethics are resolved. These issues include abortion, prenatal injury, preconception and prenatal screening for disability, genetic enhancement and eugenics generally, meat eating, climate change, reparations for historical injustice, the threat of human extinction, and even proportionality in war. Although the essays in this book address foundational problems in population ethics that were discovered and first discussed by Parfit, they are not, for the most part, commentaries on his work but instead build on that work in advancing our understanding of the problems themselves. The contributors include many of the most important and influential writers in this burgeoning area of philosophy.