The optimization of energy systems is of crucial importance for a rational use of natural and economic resources and for minimizing their adverse effects on the environment. Optimizing such systems may be considered at three levels: synthesis (configuration), design (component characteristics), and operation. The first two of these levels are ex-amined in this article. After a discussion on the uniqueness of the solution and the pos-sibility of finding this solution, the principal approaches and methods for solving the optimization problem are described in brief.