In this chapter, the effects of neutron irradiation on zirconium alloys are described. Irradiation effects on the microstructure are first discussed, describing damage creation, point-defect evolution leading to loop formation and also secondary-phase evolution and alloying elements redistribution under irradiation. Then, in the second part, irradiation effects on postirradiation mechanical behavior and deformation mechanisms, especially during tensile test and also after postirradiation thermal treatment or postirradiation creep tests, are given. Finally, the in-reactor deformation behavior, growth, and irradiation creep, as well as the related deformation mechanisms, are analyzed.