Abstract: In this paper, the ideas on "the Most Economical Control" and "the Most Economical Observation" are proposed, and the problem on the structural synthesis of the Most Economical Control System is discussed in detail. Starting from the multivariable "Harmonic Control" principle, the concepts on the "Subtypes" of controllability and observability are introduced, and the related definitions and theorems are obtained. Based on them, the direct method of the Most Economical Structural Synthesis of linear time-invarying system is given and illustrated by two examples. Then, the open problems in the theory of the Most Economieal Control Systems are mentioned.