Structural optimization is a design system for searching better solutions, which better fulfil engineering requirements. The main requirements of a modern load-carrying structure are the safety, fitness for production and economy. The safety and producibility are guaranteed by design and fabrication constraints, and economy can be achieved by minimization of a cost function. A lot of structural versions fulfil the design and fabrication constraints and designers should select from these possibilities the best ones. A suitable cost function helps this selection, since a modern structure should be not only safe and fit for production but also economic. This paper shows the elements of this system, describing optimization methods, cost calculations and an example. In the structural optimization process for an engineer it is important to know the behaviour of the structure well, the stresses, deformations, stability, eigenfrequency, damping, etc. It is as important to have a reliable optimization technique to find the optimum.