With 3,533 full-sized zero-emission buses (ZEBs) on the roads of 46 states in the U.S. and more on the way, transit agencies are looking to address the issue of keeping the bus cabin warm without experiencing significant range limitations. This CALSTART report provides insight on the current technology and use of fuel-fired heaters in transit buses and the challenges transit agencies face in pursuing zero-emission goals with a diesel-fueled auxiliary heater.