Host-rock chemical alteration and syntectonic veins in and near fault zones are evidence for episodic fracturing and fluid transport during faulting. Alteration minerals, vein fillings, and fluid inclusions may be used to estimate fault-fluid chemistry, temperature, and pressure. Fluid inclusions in thrust faults, reverse faults hosting mesothermal gold deposits, and exhumed footwall rocks of normal faults show that fluid components include NaCl, CO 2, CH 4 and CaCl 2 in addition to H 2O. Fluid composition, temperature, and pressure are spatially and temporally variable on most faults; a typical fault fluid does not exist. NaCl concentrations in fault fluids vary from 0 to 39 wt.%, CaCl 2 concentrations range up to 19 wt.% and CO 2 concentrations range up to 32 mole% in fluid inclusions, but some inclusions are present that are 100 mole% CO 2. Homogenization temperature measurements and pressure estimates confirm that these fluids were trapped at elevated pressure at depth on the faults. In CO 2-bearing fault fluids, pressures fluctuated, and a range of CO 2 contents indicate effervescence. Varying solution densities of NaCl–H 2O fluids have been interpreted to result from entrapment of fluids in inclusions at constant temperature and varying pressures. Diverse fluid compositions are present on some faults with similar homogenization temperatures and estimated pressures suggesting similar depths on the faults. Pressure, temperature and fluid composition determine the solubilities of fracture-filling minerals calcite and quartz and the formation of alteration minerals that are related to the mechanical behavior of the rock. Quartz may precipitate as a result of cooling or pressure reduction, but calcite solubility increases with cooling and decreases with decreased P CO 2 . Higher salinities increase solubilities of calcite and quartz and decrease the pH for equilibrium among feldspars, muscovite and solution. Mineral assemblages provide evidence of depressurization of the fluid as fluid moves from higher- to lower-pressured reservoirs. Precipitation of quartz, calcite, and K-feldspar or albite in fractures may result from fluid depressurization. Fault-zone rocks containing stilbite and laumontite reacted with fluid that contained little CO 2 at comparatively low temperature and pressure; kaolinite, prehnite, muscovite, epidote, and chlorite formed from fluids at higher temperature and pressure. Variations in mineralogy and fluid-inclusion characteristics on individual faults suggest separate fluids that differ in chemical composition, temperature, and pressure.