The selected letters of Walter Eucken from the years 1933 to 1945 shed light on previously unknown aspects of his biography and offer insights into a range of personal and academic relationships that were of great significance for the genesis of his work as well as for the formation and activities of the Freiburg School and the Freiburg Circle. The letters are an important source for the genesis of Eucken's two major works. They make it evident how and why, in the mid-1930s, he abandoned his plan to write a "Theoretical Economics" and instead focused entirely on the "Foundations of Economics." The letters also reveal how his work on the "Foundations" led him to the later, posthumously published "Principles of Economic Policy." For many questions concerning the history of ordoliberalism and Eucken's biography and works, the letters are the decisive source. They show his conduct during the processes of political conformity in 1933/34 and provide an impression of Eucken's assessment of the general political situation and of specific political events. In some draft letters, Eucken's criticism of Nazi policies becomes openly visible. Some letters hint at the threats he and his family faced under the Nazi regime and the depth of his engagement with the basic tenets of Nazi conceptions of science. The letters also make clear his reflections on religion and worldview and their relationship to science. The letters make it possible to trace Eucken's path toward his understanding of order and "ordo" as well as the relationship between freedom and order. It becomes evident that Eucken's and the ordoliberal school's essential conception of order and freedom was a direct result of their confrontation with National Socialism.