Background: The accurate measurement of physical activity is crucial to understanding the relationship between physical activity and disease prevention and treatment. Objective: The primary purpose of this study was to investigate the validity and reliability of the activPAL physical activity monitor in measuring step number and cadence. Methods: The ability of the activPAL monitor to measure step number and cadence in 20 healthy adults (age 34.5 ± 6.9 years; BMI 26.8 ± 4.8 (mean ± SD)) was evaluated against video observation. Concurrently, the accuracy of two commonly used pedometers, the Yamax Digi-Walker SW-200 and the Omron HJ-109-E, was compared to observation for measuring step number. Participants walked on a treadmill at five different speeds (0.90, 1.12, 1.33, 1.56, and 1.78 m/s) and outdoors at three self selected speeds (slow, normal, and fast). Results: At all speeds, inter device reliability was excellent for the activPAL (ICC (2,1)≥=0.99) for both step number and cadence. The absolute percentage error for the activPAL was <1.11% for step number and cadence regardless of walking speed. The accuracy of the pedometers was adversely affected by slow walking speeds. Conclusion: The activPAL monitor is a valid and reliable measure of walking in healthy adults. Its accuracy is not influenced by walking speed. The activPAL may be a useful device in sports medicine.