A lecture given by Hongliang Yi, at the Adventures in the Physical Metallurgy of Steels (APMS) conference held in Cambridge University. The talk is about low-density steel destined for bearing applications. Both rolling contact fatigue properties and wear resistance get improved with the increase of hardness for bearings. Large carbon content can achieve high hardness by solid solution strengthening in martensite or dispersion hardening due to carbide precipitation. Carbon decreases the martensite starting temperature and therefore increases the volume fraction of untransformed austenite that is not desired in the point view of dimensional stability since the metastable austenite could be induced to transform into martensite during service, furthermore the soft retained austenite would deteriorate the overall hardness. A novel alloy design has been proposed by high aluminium addition into the classical 52100 bearing steel to achieve the following advantages. Aluminium addition decreases the density of the steel significantly, whilst it enlarges the carbon solubility in austenite by thermodynamics and therefore more carbon is permitted to be added in this new low density bearing steel to maintain or enhance the hardness compared with conventional 52100 bearing steel. In order to validate the proposed speculation, some preliminary investigation on the designed alloy with high carbon (1.2 wt%) and high aluminium(5 wt%) will be conducted in this research, which is approximate 8 wt% lighter than the conventional 52100 alloy.