SummarySome sports carry a 50% chance of physical injury during one year. In contrast, the risk of an adult sustaining a cardiac emergency while exercising is very small, and even if such episodes cannot be prevented, the prognosis is still improved by an increase of physical activity.Since all adults should be encouraged to become more active, cost considerations dictate that simple procedures be used to screen the majority of potential exercisers.The Physical Activity Readiness Questionnaire (PAR-Q) is reasonably successful in predicting undesirable acute outcomes to exercise, with the exception of abnormal ECG responses. However, about 20% of adults fail the PAR-Q test, mainly because they have received an erroneous report that their blood pressure is high. The Canadian Home Fitness Test offers some objective basis for a personally adjusted exercise prescription; however, day-to-day variations of maximum oxygen intake and interindividual differences in the energy cost of physical activities are such that even costly laboratory prescriptions of intensity must be monitored by the individual while exercising. Accompanying this test are suggestions for specific additional precautions that should reduce exercise risks.Clinical evaluation is still far too variable, different published series showing exercise exclusion rates of 0.7 to 15.8%. There is a need for a fuller understanding of how cardiac risk factors are modified by exercise, both in the middle-aged adult and the ‘postcoronary’ patient. Sophisticated laboratory tests are not particularly helpful in the average adult; application of Bayes’ theorem shows that more than 70% of abnormal exercise ECGs are misdiagnoses relative to an angiographic criterion. Even in the high risk adult, a stress ECG will not predict the individual who will suffer an exercise-induced cardiac catastrophe. It may help in setting an intensity of exercise that the patient can tolerate, but even for this purpose the prescription will need fine tuning on a daily basis.