An expert panel workshop had two specific aims: (a) to review the current state of knowledge of existing methods for assessing physical activity and sedentary behavior in order to determine their reliability, validity, feasibility, strengths, and limitations and (b) to set research priorities and recommendations to enable the use of reliable and valid instruments for assessing physical activity and sedentary behavior within the context of three public health functions for children ages 2–5 years. Experts presented four major recommendations for research priorities at the conclusion of the 2-day workshop. The need to develop valid methods for measuring physical activity and sedentary behavior was considered the necessary first step to accomplish meaningful physical activity surveillance, public health research, and intervention research for children ages 2–5 years.