This contribution addresses the current understanding of gaseous nitriding and nitrocarburizing. Aspects of thermodynamics, kinetics and microstructure development in iron and heat treatable steel will be explained. In these materials the nitrided/ nitrocarburized case can be subdivided in a compound layer consisting of iron (carbo-)nitrides and a diffusion zone, consisting of a dispersion of alloying element nitrides in ferrite. The compound layer provides beneficial tribological and corrosion performance, while the diffusion zone is responsible for improved fatigue performance. Furthermore, aspects of low temperature surface hardening of stainless steels in a gaseous environment will be addressed. Here, the developed case consists of expanded austenite and/or expanded martensite, which essentially is a super saturated solid solution of nitrogen/carbon in austenite/martensite. The current state of the art and necessary future research activities for improvement of the present understanding will be identified.