An advanced Zircaloy cladding containing niobium has been developed and tested extensively both in long-term out-of-pile autoclave exposures and through high burnup irradiation in a pressurized water reactor (PWR) environment. Tubing of zirconium-based binary alloys containing 0.5, 1.0, and 2.5% niobium, and a quaternary composition containing tin, niobium, and iron was fabricated in such a manner that the second phase was fully precipitated, but with minimal particle growth. Autoclave testing in pure water and steam over the temperature range of 589 to 727 K indicates that all of the alloys except the 0.5% Nb have a lower post-transition corrosion rate than does Zircaloy-4, with the relative benefit increasing with temperature. Additional autoclave testing in LiOH solutions indicated a marked sensitivity of the Nb binaries to accelerated corrosion, and in these solutions only the Sn-Nb-Fe alloy was superior to Zircaloy-4. Experimental fuel assemblies with cladding of the advanced alloys were examined after one, three, and four cycles in the BR-3 reactor. Rod average burnups of up to 71 GWD/MTU were obtained with total residence times of up to 66 months. Results of post-irradiation examinations are given only for the Sn-Nb-Fe alloy as compared to Zircaloy-4. These examinations revealed that the Sn-Nb-Fe alloy showed the lowest overall corrosion, up to 50% better than Zircaloy-4 at the highest burnups, and was superior to the Zr-Nb binaries. The Sn-Nb-Fe alloy, called ZIRLO^™, also displayed lower irradiation growth and creep than the others.