The topic of this paper is the characterization of heat-treated and thermochemically treated steels. The paper itself consists of a theoretical and a practical part, along with which an analysis of the results obtained in the practical part was also carried out. The theoretical part consists of the description of the heat treatment and its parameters, the description of hardening, and the importance of the metastable Fe-Fe3C and TTT diagram for the procedure itself. Hardenability is defined as the property of steel. Other procedures for heat treatment are listed and elaborated. Also, processes of surface modification, nitriding, nitrocarburizing, boriding, and induction hardening are described. In the practical part of the paper, metallographic preparation of thermally and thermochemically treated samples was done, analysing their microstructures using a microscope and measuring the microhardness of hardened samples with different carbon contents. Finally, an analysis of the influence of carbon content on the microstructure and the measured hardness was also carried out for the hardened samples and the same samples in the normalized state.