Vibratory roller-based measurement of soil stiffness during intelligent compaction and continuous compaction control can be used for performance-based quality assurance of unbound materials. To realize this potential, the relationship between the roller-measured soil stiffness and the soil modulus, particularly resilient modulus, must be understood. The in situ stress states and paths experienced by soil beneath a vibratory roller were compared with stress states and paths during laboratory resilient modulus testing. Observed stress fields within the 1-m depth of influence varied considerably for both vertically homogeneous embankment soil and layered base over subgrade conditions. During low excitation force vibration, roller-induced levels of deviator stress were notably greater than those used during laboratory resilient modulus testing, whereas levels of mean stress were less. Predicted modulus variation with depth was strongly influenced by modulus function parameters. With typical granular soil parameters, modulus was found to be constant with depth for the embankment conditions. With modulus parameters of more fine-grained behavior, modulus increased considerably with depth.