New challenges raised almost in all segments of the global economy in the last two decades. The energy-, finance-, environmental crises, as well as the intensive use of natural resources require new design methods in the engineering activity, for both the product design and the manufacturing process planning. An effecting tool to be competitive is to apply the optimum design methods to the engineering tasks. This paper reviews the different methods of mathematical optimization, which are widely used for solving engineering problems, and describes the application of this method for two different cases. The first sample shows how to find the optimal dimensions of the welded, box type frame of a freight bogie, which will minimize the manufacturing cost of the structure and will satisfy several restrictions, e.g. mechanical stress limit, dimensional constraints, buckling, and fatigue conditions. The other case is to find the optimal geometric dimensions of a pipe insulation system which will result in a minimum investment and operating cost, when the heat loss and the outside surface temperature are restricted.