  This chapter presents a view of a non-autonomous oscillator as a mapping from
input functions of time to a circle of possible solutions (state functions of
time). It indicates how this view encompasses chronotaxic systems and enables
one, at least conceptually, to understand the extent of synchronisation in
networks of oscillators, whether autonomous or not. For the latter a
hierarchical aggregation scheme is introduced. The approach is based on the
theory of normal hyperbolicity.
