Optimal operation of power plants, electrical networks and power systems is a most important issue in conditions of the electricity market. There are many unsolved problems in the field of optimal operation of power systems and power plants which are usually tackled as deterministic ones. Actually the initial information is never complete. The information may be in several forms: probabilistic, uncertain and fuzzy. This paper presents the principles of optimum dispatch of thermal and electrical power between boilers and power units in power plants under incomplete information.