The thermoreactive deposition and diffusion process is a heat-treatment-based method to form coatings with compacted layers of carbides, nitrides, or carbonitrides, onto some carbon/nitrogen-containing materials, including steels. The amount of active carbide forming elements/nitride forming elements, coating temperatures and time, and thickness of substrates influence the growth rate of coatings. This article lists carbide and nitride coatings that are formed on carbon/nitrogen-containing metallic materials, and describes the coating process and mechanism of coating reagents. It details the growth process and nucleation process of carbide and nitride coatings formed on the metal surface. The article discusses the advantages, disadvantages, and characteristics of the various coating processes, including high-temperature salt bath carbide coating, high-temperature fluidized-bed carbide coating, and low-temperature salt bath nitride coating.