The Falling Weight Deflectometer (FWD) is most commonly used to evaluate highway pavement structural condition for estimation of pavement remaining service life and overlay design. Recently, the need to accurately characterize layer material properties of the existing pavement has increased with the implementation of the new AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG). Although a number of studies have been conducted for using FWD data in rehabilitation design, several challenges still exist due to the complex nature of pavement system. These challenges include but are not limited to: 1) the viscoelastic modulus of asphalt mixture that varies depending on temperature profile in the pavement and loading frequency; 2) the nonlinear anisotropy of unbound material; and 3) dynamic analysis of pavement behavior under FWD loading. The primary goal of this research project is to develop an integrated method for pavement in-situ condition evaluation and rehabilitation design based on modeling and analysis of FWD deflection data. To achieve this goal, the research objectives include: 1) developing an advanced model to simulate pavement behavior under FWD loading that consider viscoelasticity of asphalt layer and nonlinearity of unbound layers; and 2) assessing relationship between existing layer condition, critical pavement responses, and deflection basin parameters. It is envisioned that the proposed methodology can be used by state highway agencies in practice for assessing existing pavement condition and selecting appropriate rehabilitation strategies.