Injury surveillance is the ongoing collection of data describing the occurrence of, and factors associated with, injury. The success of any sports injury surveillance system and its wide scale applicability is dependent upon valid and reliable definitions of sports injury, injury severity and sports participation. Published sports injury reports are often difficult to interpret and compare with other published data because of different data collection and/or analysis methods. Standardised data collection methodologies including definitions are crucial for improving the comparability and interpretation of published data. Attention needs to be directed towards the definition of both risk and exposure factors since the validity and usefulness of the outcomes of research activities, data collection and surveillance systems rely on these. International consensus on appropriate definitions would greatly assist the collection of comparable and reliable sports injury data. Standardised definitions are also needed to answer questions such as: 'what is a sport? When should an activity be considered to be recreational rather than sport? Who is a sports participant? How should sports participation be measured? What is a meaningful measure of exposure to injury risk? What is a sports injury? How should sports injury severity be measured? How severe must an injury be before it should be considered to be a sports injury for surveillance purposes?' Agreed definitions and answers to these questions are essential before injury surveillance is established. Sports injury data is needed to guide injury prevention activities, to set and monitor sports safety policies and interventions, and as the basis of sports injury prevention research. All sports injury surveillance systems should therefore collect information about the epidemiology of sports injuries and their outcomes in a form that is of relevance across a broad range of potential users of the data.