In-situ polymerization is the formation of a polymer directly within or on a pre-existing phase that is intended to remain associated with the resulting polymer. Monomer, initiator or catalyst, and any crosslinker are introduced into a host such as a porous substrate, dispersed filler, fiber assembly, second polymer, or confined domain; polymerization then occurs at that location, producing intimate interfacial contact as the chains grow ^[1]. In polymer nanocomposites, this commonly means dispersing layered silicate or another nanoscale filler in monomer before initiating polymerization, so the matrix is generated around and between filler surfaces ^[2]. The defining expression “in situ” describes the locus and sequence of formation, not a distinct propagation mechanism: radical, ionic, coordination, ring-opening, or step-growth chemistry may be used. The concept excludes simple melt blending of a preformed polymer with a filler and excludes polymerization performed separately followed by impregnation, because in both cases the macromolecule is not generated within the target environment.