This Chapter presents four documents that, in different ways, illustrate Hilbert’s thinking on the foundations of physics. The first document is a set of page proofs for Hilbert’s “First Communication” on the “Foundations of Physics” (Hilbert 1915, this Volume pp. 28ff.). These proofs differ significantly from the published version, in particular with regard to Hilbert’s discussion of the energy concept and, consequently, with regard to the explicit axiomatic structure of the theory. The second document is a brief sketch of notes for what appears to be a talk on his theory of the “Foundations of Physics,” given some time after mid-December 1915, in which Hilbert concentrates on the representation of matter qua electricity in his new, generally covariant and unified theory of gravitation and electromagnetism. The third document is the typescript of a lecture, given in December 1916, on the causality principle in general relativity. The fourth document is a batch of handwritten notes for a series of lectures on space and time, delivered to German soldiers in Bucharest, Romania, in March 1918.