The data presently available on the plane strain crack arrest toughness (KIa) of steels are reviewed, using as a definition of KIa the stress intensity factor, K, a short time after a run-arrest segment of rapid crack extension. The data indicate that KIa, defined in this way, is a material property, i.e., it is independent of the manner in which it is measured. Calculations, available in the literature, on the dependence of dynamic K on a crack velocity are used to justify this invariance. It is also shown that KIa may be the minimum value of toughness, at a given temperrature, as a function of loading rate.