This Volume has three sections, General Relativity, Epistemological Issues, and Quantum Mechanics. The core of the first section is Hilberts two semester lecture course on ?The Foundations of Physics? (1916/17). This is framed by Hilberts published ?First and Second Communications? on the ?Grundlagen der Physik? (1915, 1917). The section closes with a lecture on the new concepts of space and time held in Bucharest in 1918. The epistemological issues concern the principle of causality in physics (1916), the intricate relation between nature and mathematical knowledge (1921), and the subtle question whether what Hilbert calls the ?world equations? are physically complete (1923). The last section deals with quantum theory in its early, advanced and mature stages. Hilbert held lecture courses on the mathematical foundations of quantum theory twice, before and after the breakthrough in 1926. These documents bear witness to one of the most dramatic changes in the foundations of science.