A lecture given by Alexander Chaus, at the Adventures in the Physical Metallurgy of Steels (APMS) conference held in Cambridge University. High-speed steel is inoculated with tungsten carbides to refine the structure. HSS are widely used in many metal-cutting tools, for which primarily high toughness is the main requirement providing their effective use in different cutting operations. The toughness of HSS is strongly affected by carbides formed during solidification of the melt. Melt treatment methods, particularly with addition of a small amount of surface-active or/and inoculating elements, have permitted the development of improved HSS with greater carbide control. In this work, additions of powder W and WC were used to improve the as cast microstructure of M2 HSS. Additions of up to 0.6 vol.% W and WC were made to the parent steel melted in an induction furnace. SEM and EDS confirmed that inoculation favoured the formation of the M6C eutectic instead of the M2C and VC ones, which prevailed in the parent steel. Under the action of inoculation, there was a refinement of the primary grains of the matrix and a transition from their typical dendritic structure in the parent steel to the microstructure with predominantly equiaxed morphology in the inoculated steels. The relationship between properties and microstructure is discussed.