The electric breakdown strength of ionic crystals at low and moderate temperatures is calculated on the basis of von Hippel's low energy criterion. Fr\"ohlich's method of calculation, modified to take account of the electronic polarizability of the ions, is employed. The breakdown strength at $T=0\ifmmode^\circ\else\textdegree\fi{}$ is ${F}_{0}(\frac{\mathrm{volts}}{\mathrm{cm}})=134\ifmmode\times\else\texttimes\fi{}{10}^{6}{(\ensuremath{\hbar}{\ensuremath{\omega}}_{t})}_{\mathrm{ev}}[\frac{({\ensuremath{\epsilon}}_{s}\ensuremath{-}{\ensuremath{\epsilon}}_{0})}{{({\ensuremath{\epsilon}}_{s}\ensuremath{\epsilon}_{0}^{}{}_{}{}^{3})}^{\frac{1}{2}}}](\frac{{m}^{*}}{m})$ where ${(\ensuremath{\hbar}{\ensuremath{\omega}}_{t})}_{\mathrm{ev}}$ is the reststrahl energy in electron volts, ${\ensuremath{\epsilon}}_{s}$ and ${\ensuremath{\epsilon}}_{0}$ are the static and optical values of the dielectric constant, and $\frac{{m}^{*}}{m}$ is the ratio of the effective mass to the free electron mass. The breakdown strength increases slowly with increasing temperature. The lack of dependence of the breakdown on crystallographic orientation and the strong directional dependence of the breakdown paths are qualitatively accounted for.