Current guidelines for physical activity in the early years (0-4 years) lack support by high-quality evidence. Existing studies use a large variety of measures of physical activity with limited or unknown validity. In this #Dare2Share session we aim to discuss these challenges when assessing physical activity in infants, toddlers and preschoolers: which operationalization reflects children’s activity best? Should we operationalize physical activity differently in the different age groups? What methods are suitable to assess young children’s physical activity?