Technical Abstract: A number of different activity monitors brands are available for investigators to use, but little is known about how they compare at different levels of physical activity in the field, nor if data inter-conversion between brands is possible. Over the course of 13 days, 56 women and men were fitted with two different activity monitors; Actigraph (Actigraph LLC; AGR) and the Actical (Mini-Mitter Co.; MM). Both monitors were fixed to an elasticized belt worn at the waist for the entire study period. The AGR detected a significantly greater amount of activity per day (216.2 106.2 vs. 188.0 101.1 counts/min, P < 0.0001). The average difference between monitors expressed as a coefficient of variation was 3.5 and 6.8% for log transformed and raw data, respectively. Inter-conversion of data results in a reduction in the bias and mean square error. Overall, inter-conversion of the data between brands produces excellent group mean estimates, but individual subject results must be interpreted with caution.