Good thermal and mechanical reliability of fiber Bragg gratings are indispensable requirements for telecommunication and sensor applications. The basic model of thermal reliability is reviewed and the lifetime of Bragg gratings in different germanosilicate fibers is evaluated. Very good thermal stability over 50 years can be predicted for germanosilicate fibers if the gratings are annealed for 50 hours at T Å 120°C. The mechanical reliability of irradiated fiber is shown to depend strongly on the laser source and the irradiation conditions used. KrF excimer laser irradiation decreases the mechanical resistance considerably. In contrary, frequency doubled CW Ar laser irradiation preserves the median breaking stress and reduces the maximum constant applied stress only by 6% compared with the pristine fiber for a failure time of 50 years;