Soccer (association football) is a team sport that incorporates frequent fluctuations between high and low exercise intensities. These unpredictable changes may be accompanied by unorthodox patterns of movements and the performance of specific skills. The individual activity profiles are highly variable and include elements of self-pacing, since decision making about opportunities to become engaged in play dictates individual activities. Approaches utilised to investigate the demands placed on players during competitive performances include behavioural observations during games, physiological evaluations in matches and assessments of the physical capacity of players. Observations made during games to determine the work-rate patterns of individual players are highly variable and make generalisations based on individual activity patterns conditional, unless the sample sizes are large and data are collected on a number of occasions. The data may also be affected by the diverse methodological approaches to their collection and analysis and a failure to determine the reliability and objectivity of the relevant measuring tools. Techniques that can be used to collect data in matches are limited as the sports rules and regulations restrict some approaches. The validity of applying data from non-competitive matches to the competitive situation must, therefore, be subject to formal scrutiny. There is also a concern as to the degree to which principles of steady-state are applicable to dynamically changing exercise intensities. In the evaluation of the physical capacities of players, the variability in overall soccer performance is reduced to fitness statistics, whereas in reality, soccer performance is a construct based on many different performance components and their interaction at the level of both player and team. Despite these caveats, valuable insights have been acquired into the physiological requirements of the game that have subsequently informed both research projects and impacted upon practice. The challenge for future researchers is to overcome remaining research design hurdles and devise ways to understand more fully the complexities of invasive field games such as soccer. The interactions between individuals within a team require investigation and there is a need to refine and develop methods that employ sophisticated measurement techniques and yet possess both internal and external validity, such as laboratory-based simulations.