Relative to work on nutrient intake and growth in infancy and toddlerhood, research on physical activity (PA) from birth to age 24 mo is limited. In this review, the developmental course of PA in infancy and toddlerhood is described, and the issues that surround its measurement are addressed. Of the variety of techniques that allow for gauging PA in infancy and toddlerhood, caregiver questionnaires, direct observations, and motion sensors have been used most frequently. Although each method has shown utility, the limitations of each are also acknowledged. In addition, the relation of early PA to nutrition and overweight in infants is considered. Despite the challenges to accurately monitoring early PA, its possible contribution to early excess weight gain should be recognized.