The need to accurately characterize the structural condition of existing pavements has increased with the recent development, release, and ongoing implementation of the Mechanistic-Empirical Pavement Design Guide (MEPDG), developed under National Cooperative Highway Research Program Project No. 1-37A. A number of different material inputs are required in the procedure, and it is important that these be adequately characterized and defined so that competent structural designs can be developed. The analysis of deflection data collected with a falling weight deflectometer (FWD) provides a quick and reliable way of characterizing many of the parameters of the existing pavement layers. This paper summarizes how deflection data are incorporated into the MEPDG and describes two case studies, one with a flexible pavement and one with a rigid pavement. Significant findings and recommendations from the evaluated flexible pavement case study include the following: surface-down cracking is critical in the design of the hot mix asphalt (HMA) overlay, correction factors should be used for adjusting backcalculated layer moduli to laboratory determined values, and an FWD testing frequency of 30 Hz should be used for estimating the existing HMA modulus. For rigid pavements, the case study found that the thinnest overlay produced from the MEPDG was a bonded Portland cement concrete (PCC) overlay, whereas the HMA overlay was unreasonably thick. Within the design procedure, the manually entered k-value is used for unbonded and bonded jointed plain concrete pavements but does not appear to be used by the program in the HMA overlay design. The backcalculated dynamic (or static) elastic modulus should be used for the PCC layer, and the dynamic k-value should be used for the supporting layers. The backcalculated k-value representing the composite stiffness of all layers beneath the slab does not appear to have a significant influence on the design thickness for the pavement structure analyzed.