The case developing during low temperature surface hardening of austenitic stainless steel by nitriding, carburising or nitrocarburising consists of a supersaturated interstitial solid solution of nitrogen and/or carbon in austenite. The favorable properties of this so-called expanded austenite depend on the profiles of interstitial concentration and associated composition-induced residual stress over the case. In the present contribution an overview of the current state of understanding of the evolution of the microstructure, composition and residual stress during low temperature surface hardening of stainless steels is presented. The manuscript showcases the joint achievements of many co-workers in the last 18 years. The overview concerns theoretical, experimental and modelling aspects of expanded austenite, both as a homogeneous phase and as a case on stainless steel.