This paper describes how strengthened bus systems, built on advanced bus technology and rapid bus corridors (or “bus rapid transit”, BRT) could play an important role in putting urban transport on a more sustainable path than present trends of chaotic traffic and air pollution suggest. Building on a study by the International Energy Agency, Paris, this paper provides some key results from recent efforts in Mexico City. Buses tend to carry a large share of travelers but contribute only a small part of total traffic and pollution. Transport policies that both clean up buses (reduce emissions) and move significant numbers of trips from individual modes (e.g., two wheelers and cars as well as microbuses) to large buses could provide significant relief from these problems of urban transportation. Advanced bus propulsion systems – e.g. fuel cell and hybrid buses running on a variety of fuels – could provide even greater relief from pollution problems than current commercial bus models. However, spending scarce funds for cleaning up the present technologies (through introduction of low-sulfur diesel fuel and basic emissions control equipment, or shifts to conventional alternative fuels such as CNG), along with improved bus traffic flow, would provide far more relief in the near-to-medium term than spending significant sums on a few advanced buses. Both paths must be followed, but the latter must not obscure the former. Conditions necessary for bus systems to prosper included increased speeds and capacity, and better integration into overall transport systems. We illustrate this with recent developments in Mexico City, which has opted to develop a BRT system after twenty years of collapse of its once large bus system.