Purpose: To investigate the effect of guideline operationalization in terms of intensity threshold, bout duration, and days on the proportion of children meeting the health-related 60-min physical activity guideline using a subjective and an objective assessment method. Methods: Five hundred and twenty-one children (6-11 yr) completed a physical activity diary for at least 4 d. A subsample of 51 children simultaneously wore an ActiGraph (ActiGraph, Pensacola, FL) accelerometer. Time spent above moderate-intensity thresholds of 3 and 5 METs, respectively, for continuous bouts of at least 1, 5, and 10 min was calculated. For each intensity threshold and bout duration, the proportion of children meeting the 60-min guideline was calculated. A distinction was made between meeting the 60-min threshold on each assessment day and meeting this threshold on average across all assessment days. Results: The proportion of children meeting the 60-min guideline differed considerably by guideline operationalization and assessment method. It ranged from 3% to 86% using the diary and from 0% to 100% using the ActiGraph. Overall, a higher proportion of children met the guideline when the 3-MET intensity threshold was used compared with the 5-MET threshold and when a shorter bout duration was used compared with a longer bout duration. More children met the guideline on average across all assessment days compared with the guideline on each assessment day. In general, boys were found to be more active than girls, independent of guideline operationalization and assessment method. Conclusion: Meeting the 60-min guideline highly depends on guideline operationalization and assessment method. Consensus about how the guideline should be operationalized is needed to monitor the extent to which populations of children meet the guideline and to simplify comparison, between studies Copyright © 2008 by the American College of Sports Medicine.