Abstract: Specializing in single-family residences, the firm of Willis and Lillian Leenhouts Architects was well-known for its regional modernist style and utilization of passive solar technologies. The collection includes project records of the firm (both files and drawings), as well as Willis and Lillian Leenhouts's professional papers. The project records are the most extensive portion of the collection and pertain to over 500 built and unbuilt projects from 1936 to 1990. Of the projects represented in this collection, the vast majority are single- or multi-family residences in the Milwaukee, Wisconsin area.