Correspondence; manuscripts; workbooks; diaries, research notebooks; record books. The collection is primarily correspondence (bulk dates 1930-1932) both scientific and personal. The majority of the manuscripts are original drafts and associated notes for Ehrenfest's published papers together with annotated proof sheets. The notebooks (1902-1926) contain student lecture notes, notes and comments on reading, and his own treatment of lecture subjects in later years. The diaries record impressions of all sorts (bulk dates 1903-1912, and 1922-1933). Of particular interest are the records of discussions with Bohr et al. in Copenhagen (1921-1933). The research notebooks (1902-1932) are a record of questions and problems related to his scientific thinking. Correspondents include Niels Bohr, Max Born, Johannes M. Burgers, Hendrik B. G. Casimir, Dirk Coster, Peter Debye, Gerhard Dieke, Paul Epstein, Adriaan D. Fokker, Samuel A. Goudsmit, Gustav Herglotz, Abraham F. Joffe, Ph. Kohnstamm, Hendrik Antoon Lorentz, Max Planck, B. van der Pol, A. J. Rutgers, Erwin Schrd̲inger, Jan Tinbergen, and Pieter Zeeman.