This paper deals with the interpretation of falling weight deflectometer data. Using the theory of Boussinesq and that due to Odemark, two quantities are defined in order to interpret the data: effective surface modulus; and effective subgrade modulus. The surface modulus profile serves to identify which sensor readings are suitable for backcalculation, whereas the subgrade modulus profile is used to estimate average subgrade modulus and an equivalent thickness of a pavement structure assuming that the pavement layer moduli are the same as that of the subgrade. The equivalent thickness of the pavement structure is a parameter which is directly related to the shape of a deflection basin near to the applied surface load. The variation of material stiffness with depth is studied by considering the cross-spectrum of the measured time-deflection histories. Examples are presented which demonstrate typical results from backcalculation using actual insitu data.