Background: Australia has joined a growing number of nations that have evaluated the physical activity and sedentary behavior status of their children. Australia received a "D minus" in the first Active Healthy Kids Australia Physical Activity Report Card. Methods: An expert subgroup of the Australian Report Card Research Working Group iteratively reviewed available evidence to answer 3 questions: (a) What are the main sedentary behaviors of children? (b) What are the potential mechanisms for sedentary behavior to impact child health and development? and (c) What are the effects of different types of sedentary behaviors on child health and development? Results: Neither sedentary time nor screen time is a homogeneous activity likely to result in homogenous effects. There are several mechanisms by which various sedentary behaviors may positively or negatively affect cardiometabolic, neuromusculoskeletal, and psychosocial health, though the strength of evidence varies. National surveillance systems and mechanistic, longitudinal, and experimental studies are needed for Australia and other nations to improve their grade. Conclusions: Despite limitations, available evidence is sufficiently convincing that the total exposure and pattern of exposure to sedentary behaviors are critical to the healthy growth, development, and wellbeing of children. Nations therefore need strategies to address these common behaviors.