Urban transport in most cities around the world is developing in an unsustainable fashion, as reflected by rapid growth in traffic congestion and air pollution driven by individual motorization. Strengthened bus systems, built on rapid bus corridors, and improved bus technologies could play an important role in putting cities on a more sustainable path. Results and some of the main messages are presented from a recent study at the International Energy Agency, Paris, which has assessed the situation in a number of the largest cities in the developing world, while also drawing on experience from several cities in the developed world. A principal finding is that buses tend to carry a large share of travelers but contribute only a small part of total traffic and pollution. Also, although bus ridership is declining in many cities, new types of bus systems are emerging that offer the potential to reverse these trends and have done so in several cities (e.g., Curitiba, Brazil, and Bogota, Columbia). Advanced bus propulsion systems, that is, fuel cell and hybrid buses running on a variety of fuels, could eventually provide substantial relief from busrelated pollution problems. However, in developing countries, spending scarce funds for cleaning up the emissions from present technologies, along with developing improved bus systems that enhance traffic flow, would provide far more relief in the near-term than spending on a few advanced technology buses. Both paths must be followed, but the latter must not obscure the former. Conditions necessary for bus systems to prosper are discussed and, in conclusion, recent promising developments are highlighted.