This paper presents a laboratory and numerical evaluation of interface shear strength between two hot-mix asphalt (HMA) layers. Pavement layer interface shear strength is discussed using a frictional Mohr-Coulomb-based plasticity model. The interfacial response was captured at two different loading states: pure shear and shear with compression. Direct shear testing was performed at the interface between two HMA layers to determine the effectiveness of two tack coat materials: SS-1hp and SS-1vh. Two overlays (9.5 mm and 4.75 mm nominal maximum aggregate size [NMAS]) were applied over unmilled aged nontrafficked surface. The interface shear tests were conducted at various normal pressure levels (0, 0.138, and 0.276 MPa; 0, 20, and 40 psi, respectively) at room temperature (25 C [77 F]). The measured interface shear stresses were compared with those predicted from vehicular loading in a typical thick pavement structure. A three-dimensional (3D) finite element (FE) pavement model was built to evaluate the interfacial stresses at the interface between HMA layers under moving vehicular loading. According to the results obtained from the experimental program, several parameters were found to be influential on the interlayer response. These are vehicle loading (load and tire inflation pressure) and maneuvering (braking, acceleration, and cornering). The paper also introduces the stress ratio concept for evaluating critical interface conditions. The stress ratio can be calculated based on the ratio of predicted interface stresses and interface shear strength.