Computational models at varying length scales are currently being used to improve our understanding of the behavior and performance of asphalt mixtures. The accuracy of these computational models is highly dependent on the use of accurate constitutive relationships and material constants. Typical computational models at the mastic and mortar length scale comprise asphalt binder as the matrix and aggregates as the inclusion. For such models asphalt binder is typically modeled as a linear viscoelastic material with material constants that are typically obtained using a Dynamic Shear Rheometer (DSR). This paper presents a review of the tests methods and models that are used to characterize asphalt binders. This section also includes a review of the nonlinear viscoelastic response of asphalt binders. The second part of this paper investigates in more detail the potential sources for the nonlinear response of asphalt binders and models that can be used to represent this response. A DSR was used to measure the response of asphalt binders subjected to creep?recovery at multiple stress levels and sinusoidal loading at different stress amplitudes, using parallel plate and cone and plate geometries. In addition to the shear stress and strain, the normal stress was also recorded and used in the analysis of the test results. Preliminary results indicate that asphalt binders demonstrate a nonlinear viscoelastic response even at low stress levels due to the biaxial stress state. Test data is presented to demonstrate and verify this form of nonlinear response, which is also referred to as interaction nonlinearity.