This paper discusses the heating and weakening of faults during earthquake slip, focusing on thermal processes that reduce fault strength. Field observations suggest that slip in earthquakes occurs primarily within a thin shear zone (<1–5 mm) within a finely granulated fault core. Two main thermal weakening mechanisms are proposed: (1) thermal pressurization of pore fluid within the fault core, which reduces effective normal stress and shear strength, and (2) flash heating at highly stressed frictional microcontacts during rapid slip, which reduces the friction coefficient. These mechanisms are supported by laboratory studies and theoretical modeling. Predictions suggest that strength drop is nearly complete at large slip, and melting is often precluded over much of the seismogenic zone. The paper also discusses other weakening mechanisms, such as gel formation in silica-rich lithologies, and the role of poromechanical properties of fault core rocks. Experimental data on friction coefficients at high slip rates support the thermal weakening mechanisms, showing significant reductions in friction at high slip rates. The paper concludes that thermal weakening processes are likely responsible for the observed behavior of faults during earthquakes.This paper discusses the heating and weakening of faults during earthquake slip, focusing on thermal processes that reduce fault strength. Field observations suggest that slip in earthquakes occurs primarily within a thin shear zone (<1–5 mm) within a finely granulated fault core. Two main thermal weakening mechanisms are proposed: (1) thermal pressurization of pore fluid within the fault core, which reduces effective normal stress and shear strength, and (2) flash heating at highly stressed frictional microcontacts during rapid slip, which reduces the friction coefficient. These mechanisms are supported by laboratory studies and theoretical modeling. Predictions suggest that strength drop is nearly complete at large slip, and melting is often precluded over much of the seismogenic zone. The paper also discusses other weakening mechanisms, such as gel formation in silica-rich lithologies, and the role of poromechanical properties of fault core rocks. Experimental data on friction coefficients at high slip rates support the thermal weakening mechanisms, showing significant reductions in friction at high slip rates. The paper concludes that thermal weakening processes are likely responsible for the observed behavior of faults during earthquakes.