Cast H-Series austenitic steels (HK, HP, and micro-alloyed HP) are used extensively in several industries for a broad range of high-temperature applications. The H-Series stainless steels have evolved over many years of complex alloy development that resulted in the addition of various alloying elements by trial-and-error methods. Alloy development of complex engineering alloys based on single or multiple alloying element additions or changes over wide composition ranges can often be very labor intensive, time-consuming, and expensive. Usually such traditional brute-force efforts produce only modest incremental improvements, and such improvements must then be further verified by testing relevant to real-time component service. Experience has shown that the H-series steels have reached saturation levels in both their mechanical properties and their upper use temperatures. In this talk, we present some of our recent work in developing HP-series stainless steels through use of a scientific design methodology. The native microstructure established in these austenitic alloys consists of dendritic structures of austenite matrix with finer dispersions of carbides (Cr-rich M₂₃C₆ or Nb-rich MC, depending on the alloy) in the matrix along with clusters of NbC in the interdendritic regions and dispersions of M₂₃C₆ along the seams between colonies of dendrites. Using a combination of thermodynamic modeling, microstructural characterization, and mechanical property measurements, we have derived composition-structure-property relationships for this class of alloys. With these relationships, we have successfully developed new alloy compositions with improved creep properties at higher temperatures and will be highlighted in the talk.