Many aspects of earthquake mechanics remain an enigma as we enter the closing years of the twentieth century. One potential bright spot is the realization that simple calculations of stress changes may explain some earthquake interactions, just as previous and on going studies of stress changes have begun to explain human-induced seismicity. This paper, which introduces the special section “Stress Triggers, Stress Shadows, and Implications for Seismic Hazard,” reviews many published works and presents a compilation of quantitative earthquake interaction studies from a stress change perspective. This synthesis supplies some clues about certain aspects of earthquake mechanics. It also demonstrates that much work remains before we can understand the complete story of how earthquakes work.