This article provides recommendations for the use of wearable monitors for assessing physical activity. We have provided recommendations for measurement researchers, end users, and developers of activity monitors. We discuss new horizons and future directions in the field of objective measurement of physical activity and present challenges that remain for the future. These recommendations are based on the proceedings from the workshop "Objective Measurement of Physical Activity: Best Practices and Future Direction," held on July 20-21, 2009, and also on data and information presented since the workshop.