For engineering purposes, the effects of blast waves on various target structures can be estimated quite easily with the aid of TNT-equivalence. On the one hand, many experimental data are available on the blast damage done to structures, and on the other hand, the energy content of any source can be expressed in terms of an equivalent mass of TNT. The parameters involved in the propagation of a blast wave and in its interaction with target structures are discussed at some length in order to point out the inherent inaccuracies as well as the limits of applicability of the TNT-equivalence method.