Interest in “high-temperature shape memory alloys” (HTSMA) has been growing in the aerospace, automotive, process control, and energy industries.  However, actual materials development has seriously lagged component design, with current commercial NiTi alloys being severely limited in their temperature capability.  Additions of Pd, Pt, Au, Hf, and Zr at levels greater than 10 at.% have been shown to increase the transformation temperature of NiTi alloys, but with few exceptions, the shape memory behavior (strain recovery) of these NiTiX systems has been determined under stress free conditions.  Given the limited amount of basic mechanical test data and general lack of information regarding the work attributes of these materials, a program to investigate the mechanical behavior of potential HTSMA, with transformation temperatures between 200 and 500 °C, was initiated.  In this paper we summarize the results of these studies, focusing on 1.) the practical temperature limits for NiTiX systems based on the work output of various alloys and 2.) the ability of these alloys to undergo repeated thermal cycling under load without significant permanent deflection or “walking”.  These issues are ultimately controlled by the detwinning stress of the martensite and resistance to dislocation slip of the individual martensite and austenite phases.  Finally, we will provide general rules for the design of next-generation HTSMA based on the lessons learned in this work.