This paper reviews some of the important technical barriers that must be overcome to achieve truly efficient flying adaptive micro air vehicles (MAVs). As defined by the Defense Advanced Research Agency (DARPA), MAVs are vehicles with no length dimension greater than 6 inches. These vehicles typically weigh less than 100 grams and some can fly for approximately 30 minutes. Over the past decade significant progress has been made in developing these small-scale mechanical flying machines. However, there is still much work to be done if these vehicles are to approach the efficiency and performance of biological fliers. This paper reviews the status of current miniature mechanical flying machines and compares their performance with common biological flyers such as birds, and small insects. This comparison reveals that advances in aerodynamic efficiency, lightweight and adaptive wing structures, energy conversion/propulsion systems and flight control are required to match or exceed the performance of nature’s great flyers.