In the early 1950s, the young Michel Foucault took a keen interest in the method of existential analysis—Daseinsanalyse—developed by the Swiss psychiatrist Ludwig Binswanger. He gave a lecture course on this topic at the University of Lille in the spring of 1953 and wrote a detailed introduction to the 1954 French translation of Binswanger’s Dream and Existence (1930), in which he promised a forthcoming book that would “situate existential analysis within the development of contemporary reflection on man.” This book presents Foucault’s unpublished manuscript on Binswanger and existential analysis for the first time in English, offering crucial insight into his intellectual development. Foucault carries out a systematic examination of Daseinsanalyse, contrasting it with psychiatry, psychoanalysis, and phenomenology and championing its ambition to understand mental illness. In his critique of existential analysis, Foucault began his turn toward emphasizing the primacy of experience, which would lead to the radically new perspective and genealogical methods of ‘The History of Madness’ and ‘The History of Sexuality’. Revealing a little-known influence on Foucault’s historicist approach, Binswanger and Existential Analysis reminds us of his unparalleled ability to destabilize our conceptions of self.