The in situ moduli of unbound pavement materials vary on a seasonal basis as a function of temperature and moisture conditions. The development of empirical models to predict backcalculated pavement layer moduli as a function of moisture content and stress state is addressed. The work is based on data collected via the Seasonal Monitoring Program of the Long-Term Pavement Performance Program. This research identified fundamental incompatibilities between the stress states computed from layer moduli backcalculated using linear layered-elastic theory and those used in laboratory resilient modulus testing. Important implications of this finding are that (a) application of laboratory-derived constitutive model coefficients in combination with stress states computed using linear layered-elastic theory may yield inaccurate stress-dependent modulus values and (b) meaningful advances in the state of the art for backcalculation of pavement layer moduli cannot be achieved without addressing the inaccuracies and limitations inherent in the use of linear layered-elastic theory to model nonlinear pavement response. Other important findings include the following: (c) variation in moisture content is not always the most important factor causing seasonal variations in pavement layer moduli and (d) a constitutive model form suitable for approximately incorporating the stress and moisture sensitivity of layer moduli backcalculated using linear layered-elastic procedures for practical design purposes is identified.