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Creep-recovery and stress-relaxation experiments were performed on neat and Polymer Modified Asphalts (PMAs). Through these experiments it is possible to deeply analyze these viscoelastic materials, both in linear and nonlinear conditions. The investigated functions are the relaxation modulus, G, and the creep compliance, J. As long as experiments are run into the borders of linearity, i.e. small stress and strain, simple relations can be used to describe G and J. On the contrary, in nonlinear conditions, the analytical description of G and J from stress and strain becomes extremely complex. The introduction of the so called memory function M, is one of the most used solution for the treatment of such problem and has been adopted in this study. Moreover, various linear viscoelastic models were used for the description of the creep function. The limits on the use of such models on nonlinear experiments were discussed. The dynamic creep experiment, where N consecutive creep and recovery cycles are run was also widely studied. This experiment has been recently purposed by the American National Cooperative Highway Research Program (NCHRP), as the new test for prediction of the rutting performance of PMAs. The nonlinear G(t,ã) function was studied and fitted to various linear and nonlinear viscoelastic models. The three dimensional surface of the memory function M(t,ã) was built from relaxation data and studied. The problem of separability of the memory function, i.e. the problem of the damping function, was also discussed.