This publication presents the results of research conducted on the Propylaia of the Athenian Akropolis by William Bell Dinsmoor, who began his study in 1908, and William Bell Dinsmoor Jr., who took up the project in 1962. Part I comprises William Dinsmoor's account of Mnesikles' planning of this innovative and unique structure during construction, and will be of particular interest to theorists and historians of architecture. Part II, a product of the collaboration of father and son, presents the first complete documentation of the Classical building. Supported by drawings primarily by William Dinsmoor Jr., it includes much information that is no longer accessible on the site because of restoration, weathering, or loss by other means, along with scholarly proof and detailed argumentation that support the Dinsmoors' conclusions regarding design and construction.