In this close study of a key figure in the history of technology, Leonard K. Eaton examines Hardy Cross's training, his work, his teaching, and his ideas, demonstrating how his achievements represent a pivotal moment in the history of structural engineering. During Cross's tenure at the University of Illinois (1921-37), he developed the "moment distribution method," allowing mathematicians to calculate statistically indeterminate frames of reinforced concrete for the first time. Later known as the Cross method, this achievement made possible the calculations that allowed for safe and efficient designs from reinforced concrete--a new material at the time--and the subsequent architectural revolution.