Abstract: In this paper, the (status of) several problems of hierarchical control are surveyed. The contents of this paper are as follows: In section I, the auxiliary problem and relaxation principle which are based on infinite-dimensional programming are discussed in detail. By these two principles, a unified apprach is formed, which can be used to derive several basic decomposition-coodination algorithms. In section II, the hierarchical method are used to obtain the closed-loop optimal control laws of linear and nonlinear systems; the on-line hierarchical control of the steady-state systems is also discussed. In section III, the Stackelberg strategies applied to hierarchical control are surveyed. In section IV, two algorithms of state estimation for hierarchical systems are derived.