Fluid incursion into fault zones and their deeper level counteparts, brittle-ductile zones, is examined in a number of different crustal environments. At the Grenville front, translation was accommodated along two mylonite zones and an associated boundary fault. The high- (MZ II) and low-temperature (MZ I) mylonite zones formed at 580 and 640 ¿C and 430 to 490 ¿C, respectively, in the presence of fluids of metamorphic origin indigenous to the immediate rocks. A population of posttectonic quartz veins occupying brittle fractures were precipitated from fluids with extremely negative Δ18O at 200 to 300 ¿C. The water may have been derived from downwards penetration into fault zones of low-18O precipitation on a mountain range induced by continential collision with, uplift accommodated at deep levels by the mylonite zones coupled with rebound on the boundary faults. At Lagao Real, Brazil, Archaean gneisses overlie Proterozoic sediments along thrust surfaces and contain brittle-ductile shear zones locally occupied by uranium deposits. Following deformation at 500 and 540 ¿C, in the presence of metamorphic fluids and under conditions of low water to rock ratio, shear zones underwent local intense oxidation and desilicification. All minerals undergo a shift of -10%, indicating discharge of meteoric water recharged formation brines in the underlying Proterozoic sediments up through the Archaean gneisses during overthrusting about 1000 km3 of solutions passed through these structures. At Yellowknife, a series of large-scale shear zones developed by brittle-ductile mechanisms, involving volume dilation with migration of ~5 wt% volatiles into the shear zone from surrounding metabasalts. This early deformation involved no departures in redox state or whole rock Δ18O from background states of Fe2+/&Sgr;Fe=0.72 and 7 to 7.5%, respectively, attesting to conditions of low water/rock. Shear zones subsequently acted as high-permeability conduits for pulsed discharge of >9 km3 of reduced metamorphic hydrothermal fluids at 360--450 ¿C. The West Bay fault, a late major transcurrent structure, contains massive bull quartz that grew at 200--300 ¿C from fluids of 2 to 6% (possibly formation brines). In general, flow regimes in these fault and shear zones follow a sequence from conditions of high temperature and pressure with locally derived fluids at low water to rock ratios (during initiation of the structures) to high fluxes of reduced formation or metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs. Later in the tectonic evolution and at shallower crustal levels there was incursion of oxidizing fluids from near-surface reservoirs into the faults.