During 1964–1993, researchers in polymer composites emphasized interfacial phenomena, dispersion control, and microstructural morphology as the primary determinants of performance. Methodologies coalesced around interfacial rheology, dynamic mechanical analysis, and degradation kinetics to connect processing–structure–property relationships, enabling practical design criteria and repeatable testing frameworks. A unifying thread was the drive to link microstructural features with macroscopic behavior across diverse matrices and fillers. Historical Significance: Breakthrough concepts consolidated around a three-dimensional failure framework for unidirectional composites, revealing four principal failure modes via transversely isotropic stress invariants, and informing subsequent standards and computational analyses. Parallel inquiries into particulate-filled systems demonstrated how fillers shift moduli and highlighted limits of early predictive models, prompting refinements. Investigations into dispersed phase formation in incompatible blends underscored interfacial rheology as a central lever for morphology and toughness, shaping subsequent blend design. Early kinetic analyses of char-forming plastics using thermogravimetry established robust methods to derive activation energies and kinetic parameters from heating-rate data, underpinning reliability assessments and lifetime predictions for aging polymers.