An Alloys-By-Design approach has been proposed; application of it allows compositions of single crystal superalloys to be identified using modelling methods. First order estimates of important characteristics (creep resistance, oxidation resistance, castability.) are made for alloys in a chosen compositional space. From it are eliminated those compositions which do not satisfy design constraints. The methods are being applied to design various new alloy systems against minimum property targets, e.g. An oxidation resistant Re-free (low cost) castable alloy for industrial gas turbine applications. An oxidation resistant Re and Ru containing alloy, meeting specified creep life at (i) minimum density, (ii) minimum cost. Any success depends upon accuracy of underlying sub-models and databases (e.g. CALPHAD); these need to be further improved. Still many challenges: e.g. corrosion, thermal-mechanical fatigue for which quantitative models are still needed.