ActiGraph accelerometers are a useful tool for objective assessment of physical activity in clinical and epidemiological studies. Several generations of ActiGraph are being used; however, little work has been done to verify that measurements are consistent across generations. This study employed mechanical oscillations to characterize the dynamic response and inter-monitor variability of three generations of ActiGraph monitors, from the oldest 7164 (n = 13), 71256 (n = 12), to the newest GT1M (n = 12). The response due to independent radius (22.1-60.4 mm) and frequency (25-250 RPM) changes were measured, as well as inter-monitor variability within each generation. The 7164 and 71256 have similar relationships between activity counts and radius (p=0.229) but were significantly different from the GT1M (p<0.001). The frequency responses were nonlinear in all three generations. Although the response curve shapes were similar, the differences between generations at various frequencies were significant (p<0.017), especially in the extremes of the measurement range. Inter-monitor variability was markedly reduced in the GT1M compared to the 7164 and 71256. Other measurement differences between generations include decreased peak counts and decreased sensitivity in low frequency detection in the GT1M. The results of this study revealed an improvement of the inter-monitor variability by the GT1M monitor. However, the reduced sensitivity in low count ranges in the GT1M may not be well suited for monitoring sedentary or light intensity movements. Furthermore, the algorithms for energy expenditure predictions developed using older 7164 monitors may need to be modified for the GT1M.