New high-temperature alloys are needed to replace Ni-based superalloys that have reached their maximum intrinsic temperature capability. Nb-silicide based alloys can offer balance of properties and meet property goals. In these alloys the tetragonal Nb5Si3 silicide is the key phase for achieving a balance of high temperature properties and the creep goal. In Nb5Si3 the Nb can be substituted by other transition metals, e.g. Cr, Hf, Mo, Ta, Ti and W, and the Si by other simple metals and metalloids, e.g. Al, B, Ge and Sn. The presentation will discuss the alloying behaviour of tetragonal Nb5Si3 using data for alloyed 5-3 silicides, and will demonstrate the link between alloying and the hardness and creep of Nb5Si3. It will also show how this understanding of alloying-property relationships has led to the new alloy design methodology NICE (Niobium Intermetallic Composite Elaboration) for the design and/or selection of Nb-silicide based alloys.