Multilayer coextrusion is a versatile technology to fabricate architectural multilayer polymers structures combining the properties of different materials in a single process. Understanding interfacial properties between neighboring layers is the key for suppressing the interfacial defects. Herein, we present some fundamental studies on interface/interphase in multilayered polymer systems for the better control of processing and final properties. Firstly, interfacial dynamics and rheology related to the diffuse interphase triggered from interdiffusion were studied with a model compatible polymeric system. The interdiffusion kinetics and the development of diffuse interphase were investigated by both rheological modeling and experimentally. Role of diffuse interphase at both nano- and macroscale in linear and nonlinear viscoelastic properties were elaborated. Secondly, the role of diffuse interphase in controlling the interfacial flow instability was studied in the model compatible multilayer system. Furthermore, the development of a reactive interphase with in situ formed copolymer and its effects on interfacial instabilities and the defects of coextruded films were further probed. The effect of the process parameters and the structure of the polymers on the generation of new defects called “grainy” were assessed. The macromolecular architecture of the in situ generated copolymer significantly impacted the interfacial roughness and the rheology of the multilayer stuctures. Relations between the relaxation process, elongation properties, the interfacial morphology and the copolymer architecture were established in correlation with the generation of the macroscopic grainy defect in coextrusion. Overall, our studies are aimed at uncovering the physicochemical properties of diffuse and reactive interphase to guide the better controlling of multilayer coextrussion. Lamnawar et al. Soft Matter 2016, J.Rheol. 2016, Macromolecules 2013, Encycl. of Polymer Science &Tech.2013