The purpose of this talk is to present and discuss the results to date in the development of new ultra-high strength martensitic precipitation strengthened stainless steels. These steels contain low carbon and their high strength is primarily due to the precipitation of intermetallics on tempering and the degree of strengthening is strongly dependent on cobalt content. In our initial work we have considered cobalt contents of 9, 12 ,15 ,18 and 21 wt. %. At a cobalt content of 21 wt. % we can achieve yield strengths of almost 1800 MPa. Then we will discuss compositions prepared in order to assess properties such as toughness at high strength levels. In these compositions we have examined the effects of small carbon additions and small titanium additions in addition to the effects of cobalt content. Then we will discuss the effects of composition and heat treatment variables on the Charpy impact energies and tensile properties. The heat treatment variables to be discussed will be the austenitizing temperature, refrigeration after quenching to room temperature from the austenitizing temperature and tempering temperature. Depending on the exact compositions, the refrigeration treatment and austenitizing temperature can strongly influence strength, ductility and toughness. At this point we can achieve steels with a yield strength of about 1650 MPa, ultimate tensile strengths of about 1930 MPa with tensile strains to fracture of about 0.7 and Charpy impact energies of over 30 J.