For decades, motorcycle technology lagged far behind that of automobiles. Then in 1936 Harley-Davidson introduced the Model EL, with an overhead-valve V-twin engine that was not only powerful, it was a gorgeous piece of engineering. Soon dubbed the Knucklehead for its distinctive rocker covers, the engine was so advanced that it laid the foundation for every future Harley-Davidson, and for motorcycles around the world. Illustrated with archival images or large color photos on nearly every page—including 20 two-page portraits of beautifully restored machines—this book explores the creation of this revolutionary motorcycle, as well as Harley-Davidson's wartime and postwar Knuckleheads, and its modern legacy.