Trends in urban transportation and environment are reviewed, with a focus on developing countries. "Sustainable transportation" is defined to include economic and environmental sustainability as well as equity as key criteria. Governance sustainability is also important if policies and technologies are to reduce the main externalities of urban transport. It is important to relate emissions to traffic, modal share, fuel use, and fuel characteristics; transport policies must confront all of these components if emissions are to be reduced significantly. Urban areas in developing countries have become the most polluted and congested cities in the world because of barriers to serious transport sector reform addressing these ills. However, some policies and technologies are permitting regions in Latin American and Asia to begin to change. Strong actions by cities, backed by national government formulation of equipment and fuel standards and supported by private-sector initiatives, are all needed--together with political will--to reverse the unsustainable trends in urban transport in the largest urban areas today.