How and why did plate armor come to be widely used in Medieval Europe? I trace the historical development of armor in Europe from antiquity to the middle ages, and then identify the main causes that pushed European warriors to develop and adopt plate armor from the 14th to the 16th centuries. I rely on prior research by scholars and historians of arms and armor, as well as primary source documents that describe arms and armor and their use in tournaments and on the battlefield. I conclude that a combination of social, political, military, and technical factors pushed European warriors to adopt plate armor. I also briefly discuss the demise of plate armor due to increasing use of firearms in the 16th century and the growth of professional armies.