Empirical procedures widely used in the design of flexible pavements are unsatisfactory. Difficulty is experienced in adjusting these empirical procedures to reflect improvements in construction materials and equipment. In addition, the continuous growth in traffic volumes and loads has constantly forced designers to extrapolate these procedures past the limits of reliability. The use of a multilayered elastic model to represent flexible pavement structures holds the promise of a more rational and fundamental approach to the design problem. The theoretical development in the analysis of this model is now well advanced; however a major obstacle to application of the theory is the serious lack of information which exists concerning the elastic modulus exhibited by the subgrade, base and asphalt layers in flexible pavements. In this thesis, a layered elastic model is employed to determine modulus values for these layers from plate bearing test results. The plate tests were performed on in-service » airport pavements across Canada, representing a wide range of pavement thickness, environments and subgrade soil conditions. The modulus values obtained therefore form a comprehensive set of values from which representative values and typical ranges are determined. Other aspects of these modulus values are also examined, such as the effect of testing plate sif:e on the value measured and the relationship between modulus and other material properties. In addition, the thesis explores the use of representative modulus values in conjunction with a three layer elastic model for the design of pavement thicknesses.