An explicit expression is derived for the indentation hardness numbers of alkali halide crystals in terms of ionic binding, combined with the theory of plastic indentation. The resistance to indentation is caused by the electrostatic faults that exist at the cores of {100} ⟨1¯10⟩ dislocations. The theoretical hardness is given by (cgs units)H0=0.096(e2/&egr;b4)= 1.2×10−2C44, whereeis the electron charge, &egr; is the static dielectric constant,bis the Burgers displacement, andc44is the elastic shear stiffness. The theoretical hardness for NaCl is 17 kg/mm2compared with the observed value of 16.7 kg/mm2.