Heat treatment simulation has progressed to the point where commercial software is widely available, and validations of simulation functions using experimental results have played a big role in getting here. For this reason, the author presents a number of validation cases and explains what relatively simple experiments can reveal about the complex phenomena of heat treating. In the case of validating basic functions, such as heat transfer and phase transformation, the author uses experimental results of the inverse hardening of quenched steel cylinders. When validating software at the stage where stress and strain functions are added, the author uses measurement data corresponding to length and diameter changes and residual stress distributions in normally quenched steel cylinders. Other cases presented include the validation of curving in long specimens cooled unevenly and the validation of distortions and residual stresses in carburized and quenched, induction hardened and nitrided steels.