Internal reliability, convergent validity, and construct validity of the Self-Report Habit Index (SRHI) were examined with respect to physical activity (Study 1) and sedentary behavior (Study 2) among children and adolescents. Internal reliabilities of the SRHI proved to be high in both studies. The SRHI correlated significantly with behavioral frequency measures, as well as with known cognitive associates of these behaviors. Moreover, theory-based moderating influences of habit on the attitude-intention and intention-behavior relationships were identified. The study provides early evidence to support the concept of habit as being important in dealing with physical activity in children.