During 1966–1982, seismic hazard research increasingly treated ground-motion spectra as the central organizing framework, integrating source size, energy content, site geology, and frequency-dependent behavior into unified predictive schemes. The period emphasized spectral shaping, site-adjusted hazard assessment, and metrics linking duration of shaking to energy and spectra, enabling more practical design spectra and hazard estimates. Methodological advances favored comparing competing spectral models and incorporating site effects to produce more realistic predictions across near-field and far-field contexts. Historical Significance: The era yielded foundational breakthroughs that linked source mechanics, spectral content, and site effects into a cohesive hazard paradigm. Key contributions demonstrated near-universal spectral scaling, clarified the role of effective stress in fault slip and spectra, and established duration-based and high-frequency motion characterizations that informed attenuation models and design spectra. These insights anchored later developments in spectral-based hazard modeling and site-specific design, marking a transition from purely empirical descriptions to physics-informed hazard assessment.