In this monograph, a comparative study regarding to the microstructural changes and engineering applications of martensitic stainless steels treated by nitriding and plasma carburizing reported in the literature is carried out. The changes in the microstructure and the chemical composition of the steels achieved by these thermochemical treatments are discussed by means of material characterization techniques, such as: optical microscopy, scanning electron microscopy, X-ray diffraction, optical emission spectroscopy of luminescent discharge, among others. It was found that these nitrided and carburized martensitic stainless steels are mainly used for wear resistance and corrosion resistance applications. Keywords: Microstructure, Expanded Martensite, Plasma Carburizing, Plasma Nitriding 13 INTRODUCCI