New experimental results and data from the literature are utilized to establish the upper temperature of usefulness of zirconium alloys. Three basic engineering assumptions are used: (1) service life requirements are on the order of four years; (2) tubular fuel cladding for rod-type fuel is considered with a maximum wall thickness of 0.127 cm; and (3) heat fluxes are above 157 w/cm2. The interrelation of three basic factors, corrosion rate, corrosion embrittlement by hydrogen and oxygen, and strength, are considered. An upper limit for an acceptable corrosion rate for long-term service of 1 mg/dm2 per day is set primarily by the effect of heat-transfer on corrosion. For the best alloys anticipated, this requirement (even without considering transient conditions) limits cladding surface temperatures to less that 540 C. Oxygen embrittlement of the alloy substrate by oxide film dissolution is not expected to be a limiting factor. Corrosion hydrogen embrittlement was studied in detail and found to limit acceptable service to cladding surface temperatures of less than 525 C for established experimental alloys. Hydrogen embrittlement may not be a limiting factor if alloys corrosion resistant enough to be acceptable above 600 C could be developed. Zirconium alloys designed for higher strength to overcome their inherent rapid loss of creep strength at temperatures above 540 C are expected to be more susceptible to corrosion hydrogen embrittlement. The results of this study indicate that there is good promise for developing zirconium alloys for fuel cladding application at temperatures up to 475 C.