Purpose of Review <p>This review critically examines the role of the filler-matrix interphase in resin-based composite materials, with particular emphasis on filler surface modification as an independent engineering variable. Rather than treating silanization as a fixed or auxiliary formulation step, the review aims to reposition interfacial design as a central determinant of polymerization behavior, stress development, and mechanical performance.</p> Recent Findings <p>Recent experimental and mechanistic studies consistently demonstrate that advanced interfacial strategies give rise to a functional transition region with structural, chemical, and viscoelastic characteristics distinct from both the polymer matrix and the inorganic filler. This interphase operates as a mechanical and kinetic mediator, redistributing local stress fields and enabling partial decoupling between degree of conversion and polymerization stress. Importantly, these effects appear to be strongly influenced by interfacial architecture and chemistry rather than by filler loading alone and are unlikely to be fully reproduced through incremental optimization of conventional methacrylate-based silanization alone. Despite these advances, relatively few studies explicitly conceptualize the interphase as a designable structural domain, reflecting persistent methodological and conceptual constraints within the field.</p> Summary <p>The available evidence supports a consistent conceptual transition in which the filler-matrix interphase is no longer viewed as a passive adhesion layer, but as an active structural entity influencing polymerization kinetics, stress evolution, and composite performance.</p>