Interphase developed at the polymer–polymer interface critically determines macroscopic properties of multilayered polymer systems from coextrusion process. Herein, we present some fundamental studies on interface/interphase in multi micro-/nanolayered systems to better control the coextrusion process and final properties. Firstly, the interdiffusion kinetics and development of diffuse interphase were investigated by both rheological modeling and experiment based on a model miscible polymer pair (Macromolecules, 2013, 46, 276-299). We demonstrated the effects of a robust diffuse layer–layer interphase on melt flow behaviors. Molecular rheology theories including tube model and convective constraint release (CCR) indicated a dilated tube diameter in the interphase, suggesting its weak entanglement intensity due to unfavorable interchain entanglements and its readiness to disentanglement under large deformations (J. Rheol., 2016, 60, 1–23, Soft Matter, 2017). Relating the interdiffusion, we further confirmed this finding by the existence of dynamic heterogeneity in both terminal and segmental scales, together with locally structural heterogeneity (Soft Matter, 2016, 12, 3252-3264). Secondly, micro-/nanolayered films alternating of this model miscible polymer pair were obtained by forced-assembly coextrusion process (ACS Appl. Mater. Interfaces, 2018, 10, 29019-29037). Influence of geometrical and macromolecular confinements on interfacial dynamics and rheology of multilayered systems was examined. We also revealed presence of multiple diffuse interphases and the corresponding formation mechanisms. Our findings reveal the role of interdiffusion in morphology/microstructure development, dielectric relaxations, dynamics and extensional rheology of multilayered polymers. This work will offer some new enlightenment to improve the processability of multilayered polymers and target properties by controlling the amount and length scale of interphases among layers.