Sustainable development has become a central objective of policy worldwide. Although the term is widely used, there is little agreement on what it means in practice and how progress toward it can be measured. The European Commission, as part of its Programme on Competitive and Sustainable Growth, commissioned the SUMMA (SUstainable Mobility, policy Measures and Assessment) project. Among SUMMA's objectives were to define and operationalize the concept of sustainable transport and mobility in terms of its environmental, economic, and social dimensions, and to define outcome indicators from the transport system that can help policy makers monitor progress toward its achievement. To achieve these objectives, we used the systems approach, which identifies the interrelationships among the elements of a complex system and helps in the design of policies to steer the system toward sustainability. This article describes the first phase of the systems approach: the set of outcome indicators that we developed and how we defined the transport system in terms of three markets—a movement market, a transport market, and a traffic market—in which choices are made that result in traffic streams. The traffic streams determine the values of the outcome indicators, which can then be used to identify good policies. The approach is illustrated with examples from the SUMMA project.