We addressed the following questions: How can environmental impacts of transport be identified? How can the impacts be represented by operational indicators? How can several indicators be jointly considered? And how can indicators be used in planning and decision making? We firstly derived a definition of "indicator of environmental impact in the transport sector". The concept of "chain of causalities" between a source and a final target is developed, as a common reference for indicators and their assessment. We derived criteria and methods for the assessment and selection of indicators, in terms of measurement, monitoring and management. Finally, the application of the criteria is exemplified for individual indicators of seven chains of causalities and for selected aggregated indicators.