Studies of the association between physical activity and breast cancer have yielded inconsistent findings. These findings may be related to a true null association or an inability to measure physical activity with enough precision to measure a protective relation. The authors reviewed and critiqued physical activity measurement methods used in published studies of the association between physical activity and breast cancer. The authors examined the quality of physical activity measures in 20 published studies. A summary score was created to rank the quality of the activity score. Studies with higher scores had a more precise measure of physical activity. Physical activity measurement methods were different in each study. Activity was measured by job classification, occupational tasks, participation in competitive athletics, and recreational and leisure-time pursuits. The recall period for physical activity ranged from a lifetime to the past year. Comparison of quality scores showed no associations between the precision of activity measures and the study results. Future studies of physical activity and breast cancer should utilize standardized methods to measure physical activity. Researchers should be encouraged to choose a measure based on hypotheses regarding physical activity and breast cancer mechanisms. Studies also should extend to subgroups of women with differences in other breast cancer risk factors, such as body mass, menopausal status, and hormone replacement status.