Correspondence contains about 9,000 pages and ranges over his entire career. It includes substantial exchanges with Rudolph Carnap, Ernst Cassirer, Philipp Frank, Sidney Hook, Paul Oppenheim, Wolfgang Pauli, and others. There is also significant correspondence with thinkers like Bergmann, Dingler, Dubislav, Grelling, von Laue, Morris, Nagel, Planck, Schrd̲inger, Tarski, and Zilsel. The correspondence provides not only valuable information about Reichenbach's own philosophical development but also important background material for research into one of this century's most influential philosophical movements: Gesellschaft f r empirische Philosophie (the Berlin Circle). Collection also contains substantial amount of notes and lecture outlines, including lectures from Einstein, Hilbert, and Planck. Also notes from Reichenbach's own lectures and presentations on topics in the natural sciences, mathematics and philosophy, as well as social issues and popularizations of relativity theory and quantum mechanics. Reichenbach's library, of over 1000 volumes, is also part of the collection.