The failure behavior of ceramic windows for gyrotron tubes is governed by the inert strength and, in the presence of subcritical crack growth, by the parameters of the crack growth power law. A number of ceramic materials (aluminas of different impurity, alumina with zirconia, aluminum nitride, and sapphire) have been investigated. The results are reported as Weibull plots for the strength, ν-K curves for the subcritical crack growth behavior, and da/dN-ΔK curves for cyclic fatigue. Relations, necessary for lifetime predictions, are compiled at the end.