Satisfactory synthesis of controllers for large-scale systems must necessarily be conducted within a decomposition-decentralization framework to overcome the twin problems of high dimensionality and constraints on the available information structure, and the determination of controllability conditions are useful in such designs. Now concepts of controllability based primarily on the degree of permitted information exchange among the various parts of the system, are introduced and conditions under which completely decentralized and multilevel controllers can be synthesized are developed. These control schemes are shown to possess additional beneficial features such as tolerance to structural perturbations and provision of efficient means of handling time delays caused by information transfer.