Correspondence, 10 boxes of manuscripts, workbooks, diaries, 190 notebooks, record books, family documents, and miscellanoues items. The collection is primarily correspondnce (bulk dates 1930-1932), both scientific and personal. Manuscripts consists of original drafts and associated notes for Ehrenfest's published papers together with annotated proof sheets, a few unpublished papers and his notes on lectures by others including Arnold Sommerfeld and Enrico Fermi. 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, mainly in the periods 1903-1912 and 1922-1933. Of particular interest are the records of discussions with Bohr and others in Copenhagen, 1921-1933. The research notebooks, 1902-1923, are a record of questions and problems related to his scientific thinking. Correspondents include Niels Bohr, Max Born, Johanes M. Burgers, Hendrik B. G. Casimir, Dirk Fokker, Samuel A. Goudsmit, Gustav Herglotz, Abram F. Ioffe, Ph. Kohnstamm, Henrik Atoon Lorentz, Max Planck, B. van der Pol, A. J. Rutgers, Erwin Schrd̲inger, Jan Tinbergen, and Pieter Zeeman.