Zirconia-based ceramics have long been used to provide thermal protection to the structural components of modern gas turbine engines. Economic and environmental considerations have motivated higher engine operating temperatures (>1200ºC), thereby demanding more reliable performance of such thermal barrier coatings (TBCs) over an even wider temperature range. Recent work has shown that, in these temperature regimes, microstructural and phase evolution may limit the performance of commonly employed TBC systems, such as yttria-stabilized zirconia (YSZ). However, lessons learned from the YSZ system can be applied to optimize novel TBC material systems. Development of these systems hinges upon collective improvement of an array of intimately coupled materials properties. Of these, phase stability is key to maximizing both longevity and engine operating temperature. Current schemes for improving phase stability without sacrificing other important materials properties, like toughness, will be explored.