Background: A recent approach to increasing physical activity levels, managing weight and improving health has been via technological advances, such as the Web 2.0 technologies, wearable activity trackers and smartphones. These approaches might be effective due to reduced cost, userfriendly environment and real-time feedback provided. Many of these monitors and smartphone applications are marketed to provide personal information on the level of physical activity, however little or no information is available regarding their validity.Aims: The purpose of this study was to compare the criterion validity for distance travelled and total energy expenditure (TEE) between a commercially available GPS monitor, Garmin Forerunner 310XT, and a freeware GPS application for Android smartphones, Runkeeper, under semistructured activity settings.Methods: A single, healthy and physically active participant took part in all trials. The same protocol was repeated on 40 occasions (20 walking and 20 running sessions). The participant wore the Garmin GPS on the left wrist and a smartphone with the Runkeeper application activated on the left arm. Distance was compared against an objectively measured distance with three different methods and energy expenditure estimates for each monitor was evaluated relative to criterion values concurrently obtained from the portable metabolic system Cosmed K4b2. Differences from criterion measures were expressed as a mean absolute percent error and were evaluated using repeated measures ANOVA and Bland-Altman plots.Results: For overall group comparisons, the mean absolute percent error values for distance were 0.30% and 0.74% for Garmin (walking and running), while higher values were calculated for Runkeeper (3.28% during running and 4.43% during walking), which significantly overestimated distance in both conditions. For energy expenditure estimation, significant differences were observed for both monitors (pB0.001). Garmin significantly underestimated energy expenditure compared to the criterion method in both conditions by 17%, while Runkeeper significantly overestimated it by 6.29% during running and 35.52% during walking.Conclusions: The present study offers initial evidence for the validity of GPS technology of wearable activity monitors and smartphone applications for measuring distance travelled. However, estimates of energy expenditure were poor, except for Runkeeper during running which provided acceptable error.