Selecting and correctly heat treating an appropriate tool steel for production a die is not a simple task. This paper points out defects which are encountered most frequently in dies when a poor-quality material or an inadequate heat treating schedule was selected or when the prescribed heat treating schedule was not followed. The most common deficiencies in semi-finished products from tool steels for die making include carbide banding and non-uniform microstructure with substantial variance in grain size. Incorrect tempering parameters are often used and the materials are thus treated in the region of temper brittleness. Cases of the above kind have been investigated by the laboratory at COMTES FHT a.s. in recent years and their summary is given in the text below.