There are many different notions of complexity. However, complexity does not have a generally accepted universal concept. It is becoming more clear that the search for the universal concept must be done within a final theory. In this chapter a concept of structural complexity for the first time suggests the real opportunity to search the universal concept of complexity within a final theory. Experimental facts given in this chapter allow to suggest a general optimality condition of cooperative agents in terms of structural complexity. The optimality condition says that cooperative agents show their best performance for a particular problem when their structural complexity equals the structural complexity of the problem. According to the optimality condition to control a complex system efficiently means to equate its structural complexity with the structural complexity of the problem.