Y-J. Wei, P.E. Caines This work presents a new formulation of the aggregation of the state sets of large or complex (finite input, state, output) dynamical systems. Given any control problem for the original base system the resulting hierarchical control system recursively solves a set of simpler problems for systems with simpler dynamics at each of its hierarchical layers. The fundamental notion here is that of dynamical consistency (DC) between an ordered pair of blocks of an aggregation (partition) of the set of system states. The DC property is said to hold when all the states of the first specified block may be driven directly (by some initial state dependent control sequence) into the second block. The concept of dynamical consistency permits us to define the natural notions of in-block and between-block controllability, and of a hierarchical lattice of increasingly aggregated sub-systems of a given system. Such configurations are naturally expressed in terms of hierachies of COCOLOG control languages called HICOLOG systems and the conceptual framework extends naturally to hybrid control systems.