Several process algebras for modelling hybrid systems have appeared in the literature in recent years. These all assume that continuous variables in the system are modelled monolithically, often with the differential equations embedded explicitly in the syntax of the process algebra expression. In HYPE an alternative approach is taken which offers finer-grained modelling with each flow or influence affecting a variable modelled separately. The overall behaviour then emerges as the composition of these flows. This approach is supported by an operational semantics which distinguishes states as collections of flows and which is supported by an equivalence which satisfies the property that bisimilar HYPE models give rise to the same sets of continuous behaviours.