This chapter provides an overview of complex health behavior, including defining common physical activity domains in youth, such as active play and leisure activities, active transportation, organized sport participation, and physical education. The number of times or how often physical activity is performed. For example, the number of times physical activity is performed per week, per day, or per hour. Physical activity is usually categorized into three main intensities: light, moderate, and vigorous. Research in youth often combines moderate and vigorous intensities into one category called moderate- to vigorous-intensity physical activity. These different intensities of physical activity are typically defined in terms of energy cost using a metabolic equivalent of task (MET) value, with 1 MET equaling the energy required at rest. Common types of physical activity included in public health guidelines, in particular for those aged 5— 17 years, include aerobic and resistance activities.