Two new computer programs, LEAD and FEAD, have recently been developed at Nottingham for the back-calculation of elastic stiffnesses of pavement layers from deflections measured with the Falling Weight Deflectometer (FWD). These programs represent a significant improvement on the program PADAL, previously developed at Nottingham. A revised algorithm for the iterative back-calculation process, based on the Gauss-Newton method, has been successfully implemented. Appropriate stress-strain relationships for the granular layers and the subgrade were built into the programs, enabling the non-linear behaviour generally exhibited by these materials to be evaluated. The influence of overburden, pore pressures and “locked-in” horizontal stresses on the in situ stiffnesses is also considered. Additionally, a rigid bottom may be included in the analysis. The main difference between the two codes is in the sub-routines adopted for the computation of deflections. LEAD makes use of layered elastic analysis, while FEAD uses a finite element approach which, although more accurate for non-linear problems, is more time consuming. Data from FWD surveys have provided a means of validating the new software. Examples are presented, including comparisons with laboratory test data. The results suggest that the stress-dependent nature of the lower layers can be of significance for the overall behaviour of the pavement.