Various ethical theories recommend developing a morally sound character, and therefore require an understanding of the nature and development of traits. Philosophers usually accept the Aristotelian view that traits are a combination of habit and insight. Sartre's early work offers an alternative: traits consist in projects. One aim of this paper is to show that this is indeed Sartre's view, by explaining the errors that have lead philosophers to ignore his theory of character or deny that he has one. The other is to clarify the theory, by investigating his notion of project and its relation to other prominent aspects of his philosophy, by distinguishing a broadly Sartrean theory of character from Sartre's own brand of this theory, and by comparing this broadly Sartrean theory with the Aristotelian theory. The paper focuses on Being and Nothingness and The Transcendence of the Ego, but also discusses some of Sartre's other works.