This paper deals with the characterization of pavements and their supporting subgrade by comparing the measured dynamic impedance of a site, based on falling weight deflectometer data, with that of a two-layer, elastodynamic model. The pavement is modelled as a Kirchhoff plate and the subgrade as an incompressible, semi-infinite, elastic half space. The impedance of the two-layer problem is developed in graphical form as a function of a dimensionless angular frequency that depends on the pavement and subgrade properties. The characterization methodology outlined is applied to both simulated and actual field data. The effects of bedrock location and increasing subgrade stiffness with depth on dynamic impedance are addressed, and some limitations associated with the back calculation of system parameters are discussed. Key words: pavements, layer moduli, impedance, dynamic, back calculation.