For more than two decades, many smart, dedicated people in the General Aviation industry have been working towards finding a way to eliminate the tetraethyl lead (TEL) additive in aviation gasoline. The bulk of that effort has been spent toward the goal of finding an unleaded avgas that could be easily approved by its compliance with the current ASTM specification for 100LL avgas, ASTM D910. Many fuel blends have been tested by industry and government regulators, but none compatible with the existing 100LL specification has demonstrated comparable or adequate performance. More recent research has necessarily expanded beyond D910 compatibility and has yielded some promising fuel candidates. But even if they demonstrate adequate performance, properties, and cost, the path to a mass certification cannot be achieved by D910 compliance, and therefore is now much more complicated and uncertain.