Sedentary behaviour has been identified as a specifically deleterious personal practice on multiple levels of health of individuals at all ages, including the pediatric population (i.e., 0 to 18 years of age). The aims of this paper are to (I) summarize the literature concerning the definition, the characteristics, the context, the determinants and the consequences of sedentary behaviour among children and youth; (II) propose a conceptual model that summarizes these findings; and finally (III) highlight research gaps in the literature. Sedentary behaviours are common and complex behaviours that can potentially affect the health of children and youth on the physiological and psychological levels. Those deleterious effects on health can vary depending on how they are accumulated throughout the day, for example with or without interruption, or in which context. While childhood engagement in sedentary behaviours is too high in the majority of countries where it has been assessed, developing research and policies that target the reduction of sedentary behaviours among children and youth must be ranked as a top priority for all public health organizations worldwide. Although further research is needed concerning the identification of the specific determinants and consequences of different types and patterns of sedentary behaviours in various contexts, the Conceptual Model for the Study and Understanding of Children and Youth’s Sedentary Behaviour, which I have proposed in this paper, gives an overview of the topic and supports the development of policy and further research.