Consists of Stillwell's engineering reports, correspondence, photographs, blueprints, maps, and printed matter. The engineering reports include as subjects the design and building of power plants and the electrification of railroads, some with accompanying photographs and blueprints. Stillwell's professional correspondence relates to his work as an electrical engineer with the Westinghouse Electric & Manufacturing Corporation (1886-1897), the Niagara Falls Power Company (1897-1900), and the Rapid Transit Subway Construction Company of New York (1900-1905), and, later, as consulting engineer with the Lehigh Valley Coal Company, the Paupack Water Power Project, the Lehigh Coal and Navigation Company, the Lehigh Navigation Electric Company, the United Railroads and Electric Company of Baltimore, and the Baltimore and Ohio Railroad. Also included are personal papers, such as correspondence, account books, estate matters, and material on Princeton where Stillwell served as trustee (1920-1941).