Analysis of structures has followed a generalized Newton’s 2nd law of motion since early times. Simply stated, the analyst would estimate some properties of the structure under consideration, and the exciting entity (generally in form of a force). Using a generalized equilibrium equation (which is a generalization of Newton’s 2nd law of motion), the structural response is computed. Obviously, the accuracy of such an approach depends on the accuracy of the estimation of the structural properties. Given the complexities of modern structures and the great demands of natural and manmade hazards, it is clear that traditional estimates of structural properties needs to be improved. The high costs of construction, rehabilitation, and failure of infrastructure highlights the need for an even higher degree of accuracy of estimating (identi‘cation) the structural properties.