As nickel-base single crystals are being implemented in gas turbine engines operating at temperatures in excess of 1600K, maintaining structural properties at these elevated temperatures has become an increasingly important problem. Typically, elevated levels refractory alloying elements are added to these single crystal alloys to enhance the degree of solid solution strengthening within the microstructure. The presence of Re, W, Mo, Ta in Ni-base superalloys strongly influences parameters, such as the stacking fault energies, lattice misfit, and shear modulus of the crystalline lattices, that govern the creep response of the material. However, this is a challenge when dealing with high refractory alloys that are susceptible to the precipitation of deleterious topologically-close-packed (TCP) phases. Recent investigations have demonstrated Ru additions to be beneficial with respect to hindering the formation of these TCP phases and consequently improving the creep properties of these advanced single crystal alloys. Substantial effort has been dedicated towards understanding the fundamental effects and the mechanisms by which Ru additions alter the structure of the γ and γ' phases. Much of this knowledge has been effectively applied to the development of a new class of single crystal Ni-base superalloys with improved strength and a temperature capability significantly higher than those of existing "second" or "third" generation alloys. Nevertheless, optimization of the alloy chemistries remains notoriously difficult as various component design requirements related to the solidification characteristics, overall blade density, coating compatibility and cost must also be carefully considered when developing these materials. This investigation addresses some of the issues associated with the development of Ru-bearing, high refractory content Ni-base superalloys and describes how interrelationships between the properties, processing, microstructure and chemistry of these alloys can be balanced to yield a practical "fourth" generation single crystal superalloy.