The training of competitive athletes can be assessed by retrospective questionnaires, diaries, physiological monitoring and direct observation of training behaviour. Questionnaires represent the most economical, most comprehensive and least accurate method. Diaries are more valid, but their drawbacks for long term quantitative studies are poor compliance and difficulties in processing the data they generate. Physiological monitoring (of oxygen consumption, heart rate or blood lactate concentration) provides objective measures of training intensity, and direct observation gives valid measures of most aspects of training; however, these methods are impractical for continuous, long term use. Coaches and athletes quantify training for purposes of motivation, systematisation of training and training prescription, but there has been little study of the use of training quantification by these practitioners. Motivation and systematisation are probably achieved best with diaries. Direct observation appears to be the best method of ensuring compliance with a training prescription, although heart rate monitoring is also a promising method for prescribing endurance training intensity. Sport scientists quantify training to study its effects on the performance and health status of competitive athletes. Most studies have been descriptive rather than experimental, and unvalidated questionnaires have been the predominant method of assaying training. The main areas of research include performance prediction and enhancement, overtraining, reproductive dysfunction, injury, illness, and nutritional status. Training has substantial effects in all of these areas. There is a need for more experimental studies that utilise validated measures of training to investigate how to reduce sports injuries and enhance competitive sports performance. More attention could also be given to methodological issues of training quantification.