This chapter highlights the fact that every steel casting should be heat treated. The structure of cast steel in as-cast conditions requires changes – this applies to virtually all types of cast steel grades. As a result of heating and cooling (according to various technological options), beneficial changes in mechanical and also other properties (e.g. hardness, corrosion resistance and wear resistance) are obtained. This chapter includes the types of heat treatment of cast steel, possible practical effects after each treatment and the structural changes caused during its course. There are examples of the microstructure in as-cast conditions, after quenching and after tempering at various temperatures.