Background: Physical Activity (PA) is important for maintaining health, but there are fundamental unanswered questions on how best it should be measured. Methods: We measured PA in the Netherlands (n=748), United States (n=540), and England (n=254), both by a 7 day wrist worn accelerometer and by self-reports. The self-reports included a global self-report on PA; and a report on the frequency of vigorous, moderate, and mild activity. Results: The self-reported data showed only minor differences across countries and across groups within countries (such as different age groups or working versus non-working respondents). The accelerometer data, however, showed dramatic differences; the Dutch appeared to be much more physically active than Americans and English (For instance, among respondents 50 or older only 5% of Americans and 3% of English were active enough to make it into the highest activity quintile of the Dutch distribution). In addition, accelerometer data showed a sharp decline of PA with age, while no such pattern was observed in self-reports. The differences between objective measures and self-reports occurred for both types of self-reports. Conclusion: It is clear that self-reports and objective measures tell vastly different stories suggesting that across countries people use different response scales when answering questions about how physically active they are.