The Spanish Civil War, 1936-1939, marked the full integration of airpower into warfare and introduced the latest developments in aviation technology. Whereas the First World War featured dogfights between wooden biplanes, the Spanish Civil War featured air combat between metal monoplanes. Aircraft with retractable landing gear began to replace those with fixed gear. Bombers dropped incendiaries with greater effect. Anti-aircraft weaponry increased in accuracy and range. These and other changes reflected the trends of aviation technology generally, and they foreshadowed the characteristics of aviation in the Second World War. While factors such as leadership, training, unity of effort, tactics, and overall strategy were important to the eventual outcome of the Spanish Civil War, the effective use of airpower had a critical influence and marked a dramatic change in warfare. The pursuit of superior performance in aircraft and weaponry was characteristic of the technological arms races which continued throughout the rest of the twentieth century. American observers were present in Spain in various capacities, and bore witness to the unfolding of these developments. Yet, while the trends in aviation were evident, it was unclear whether they would bring success in future wars. This thesis will examine these concepts in regards to the major powers that intervened in the civil war: Germany, Italy, the Soviet Union, and France.