This dissertation is a study of Martin Heidegger’s understanding of the concept of authenticity, and how that understanding may have changed over time, via a careful investigation of Heidegger’s written works, from Being and Time in the 1920s to works of the mid-1960s. This study has two goals. Narrowly, it argues that the key elements of Heidegger’s account of authenticity do not change over the period under consideration—while the nature of his elaboration shifts dramatically, the key content stays the same. Broadly, since a fundamental change in the understanding of authenticity, especially how it relates to history and to human volition (referred to below as the active/passive dimension), is central in most interpretations that argue for a strong turn, or Kehre, in Heidegger’s later works, this study provides substantial evidence that there was not nearly as dramatic a shift in Heidegger’s views as many argue.Chapter 1 introduces the issue by discussing Heidegger’s understanding of authenticity ingeneral, ways history and the active/passive dimension could relate to authenticity, the general outlines of strong interpretations in relation to these two issues, and what various interpreters who hold such interpretations argue. Chapters 2, 3, and 4 examine Heidegger’s views on authenticity, history, and the active/passive dimension, in roughly the 1920s, mid to late-1930s, and 1940s-1960s, respectively. Chapter 5 concludes, discussing in broad terms why strong interpretations of the Kehre are tempting even if misguided, and sketching a reading of the Kehre better suggested by the views argued for in this study.