This thesis examines the “Gill System,” a concrete wall system developed by Southern California architect Irving J. Gill (1870–1936) between 1919 and 1922. While existing scholarship on Gill largely emphasizes his early Modernism and experiments in social housing, few studies have been conducted on his construction innovations. Through both archival research and field survey, this thesis uses four case studies—Gill’s 1921 Horatio West Court, 1921 Morgan House, 1921 Reed House, and 1922 Clarke House—to verify and investigate the actualization of the “Gill System.” The thesis traces Gill’s career in concrete construction and his adaptation of techniques pioneered by contemporaries such as Julius Kahn and Robert Aiken, as well as the development of his own improvements to those systems, to arrive at a unique method of construction, the “Gill System.” Gill’s only description of the method was in a 1929 letter to Richard Neutra. Gill described a double concrete wall created by slip-form construction, sandwiching an inner waterproof membrane nailed to a wooden lattice frame. This is the only surviving first-person account of the “Gill System,” offering rare technical detail. Using the letter and case study physical evidence, the research verifies Gill’s methods and contextualizes the system within his larger career. Recognizing it as the culmination of Gill’s persistent experimentation with concrete, this thesis contributes to the developing field of modern concrete architectural heritage.