This document provides information and resources to policy makers in developing and transition countries on key steps that can be taken to clean up the emissions from bus fleets. The focus of this report is on bus fleets but the strategies detailed here can also be applied to reducing emissions from other heavy-duty diesel vehicles that operate within developing and transition countries including refuse trucks, dump trucks, or highway maintenance vehicles. A brief introduction is also provided to bus rapid transit (BRT) systems that employ clean transit buses. These BRT systems are gaining in popularity in many large cities around the globe as densely populated urban centers try to provide clean, efficient mass transportation options for its citizens.