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Laboratory modulus tests were performed on samples taken from three verification sites. COMDEF consistently outperformed the general purpose program in both speed and accuracy. A sensitivity study is presented which includes solutions using COMDEF and a general purpose backcalculation program. The use of a representative basin is discussed, and data are presented which indicate that this approach should not be used for composite pavements. Lack of knowledge about the degree of cracking in the rigid layer is a major problem, and backcalculated modulus is presented as a way to assess the degree of cracking in the rigid layer. This paper discusses the role of nondestructive testing in pavement evaluation and the particular problems in evaluating composite pavements. A description and flowchart of the method used by COMDEF is included. However, the COMDEF approach applies interpolating functions to a data base of precalculated solutions, so that the comparison basins are calculated quickly with reasonable accuracy. COMDEF, like most backcalculation algorithms, compares measured deflections with theoretical solutions. The program uses a new data base technique to speed program execution. A program called COMDEF has been written for use in nondestructive evaluation of composite pavements.