During the eighteenth century, German philosophers wrote on a broad range of topics in moral philosophy: from meta-ethical issues such as the nature of obligation, to elaborate systems of normative ethics (often in the form of a doctrine of duties to self, others, and God), to topics in applied ethics such as the permissibility of the death penalty and censorship. Moral philosophy was also intimately related to the modern natural law tradition at the time, as well as to discussions taking (...) place in theology and psychology, to name only a few other disciplines. As a result, it is often hard to isolate eighteenth-century German moral philosophy without addressing a whole host of other philosophical subjects. There are nonetheless certain central topics that were discussed by nearly every major figure of the period and which therefore serve as a window into how German philosophers approached moral philosophy during the eighteenth century. In this chapter I illustrate that one such topic is the principle of morality; namely what philosophers took to be the supreme norm at the foundation of judging actions to be morally good or evil. As we will see, although several figures approached this principle in a broadly similar way by identifying “perfection” as the central concept at work, they differ considerably with respect to important details. Furthermore, certain common threads reveal themselves: philosophers differ not only with respect to the content of the principle, but also with respect to the way in which it can be derived, how particular duties can in turn be derived from the supreme principle, and how the principle can be formulated. I begin by outlining Christian Wolff’s influential principle of perfection (Section 1), followed by a consideration of Alexander Gottlieb Baumgarten’s subtle revision of this principle (Section 2) and Christian August Crusius’s theological alternative (Section 3). I then briefly consider the impact that the reception of eighteenth-century British philosophy had on German moral philosophy during the second half of the eighteenth century (Section 4), before considering Moses Mendelssohn’s and Immanuel Kant’s answers to the 1763 Prize Essay Question (Section 5) and Johann August Eberhard’s unique mixture of rationalism and empiricism from the early 1780s (Section 6). I conclude (Section 7) with a brief discussion of Kant’s approach to the principle of morality and the principle of happiness proposed against it by his early empiricist critics. (shrink)