The main objective of this paper is to consider time-varying linear systems within the framework of the inclusion principle. Starting from general definitions, conditions are derived for a dynamic system to include a dynamic system of smaller dimension. Particular attention is paid to restriction and aggregation, the most important special cases of inclusion. It is proved, using geometric arguments within the general time-varying context, that any inclusion relationship on a given time-interval can be decomposed into a sequence of restriction-aggregation pairs. Connections between inclusion and zero-state equivalence are discussed. The paper also presents a formulation of the contractibility (expandability) conditions for time-varying state-feedback controllers.