The Papers of Olin J. Eggen shed considerable light, not only upon the professional career of a native Wisconsin astronomer and double-alumnus of the University of Wisconsin-Madison, but more importantly, upon large-scale developments which have characterized post-war astronomy and astrophysics, including the creation of large (4-meter-class) optical telescopes in the southern hemisphere. Over a career of fifty years, Eggen became an extremely productive and heavily-cited researcher as well as a noted administrator. Among the significant documents preserved in this collection are the extensive letters exchanged between Eggen and Caltech astronomer Allan R. Sandage, along with the draft of Eggens most notable scientific paper, referred to by the initials of its authors as, ELS.