Peak flow monitoring of asthma came into vogue with the advent of asthma self-management programs. Because it offered an objective way to gauge asthma severity, it promised improvement in the accuracy of asthma monitoring over that attainable by symptom monitoring. This promise has not been fulfilled. The ensuing years have witnessed a debate concerning the relative merits of symptom and peak flow monitoring. The debate has focused both on the degree to which peak flow and symptom scores are related to one another and on the relative effectiveness of symptom and peak flow monitoring for asthma control. We review research relating to these topics. The work shows that the strength of the relationship between peak flow and symptoms is low to moderate and varies between individuals and that benefits of peak flow monitoring in asthma self-management provide, at best, no more than a small increment in effectiveness beyond that afforded by symptom monitoring.