ABSTRACT: A wide array of accelerometer-based activity monitors has been developed to facilitate objective monitoring of physical activity behaviors, but it has proven difficult to equate outputs from different monitors. On the surface, commercially available monitors seem to be performing the same basic task—monitoring total body acceleration. However, differences in sensor properties and internal data processing have made it difficult to directly compare output from different monitors. In recent years, many new competing technologies have been released into the market, compounding the challenge of evaluating monitor equivalency and the relative strengths and limitations of different monitors. To advance physical activity assessment and improve our ability to compare results across studies using different monitors, it is important to conduct functional equivalency studies in a standardized and systematic way. This article summarizes issues associated with monitor equivalency and proposes methods for standardization and quality control in future research.