Fluids are central to a wide array of fundamental lithospheric processes and properties, including mass transfer, heat transfer, reaction progress, the rheological behavior of rocks and seismicity, crustal anatexis, arc magmatism, ore formation, the release of greenhouse gases, such as CO2, and the impact of such gases on the long-term carbon cycle. Pelitic sediments and hydrothermally altered oceanic crust will typically lose 2–4 wt% H2O from low to high metamorphic grades. Losses from serpentinized ultramafic rocks can exceed 10 wt%. Devolatilization of metacarbonate rocks can liberate 10–20 wt% CO2 during collisional orogenesis. Degassing intrusions and the mantle are additional fluid sources. The fluids flow pervasively between grains or are channelized into structures, such as fractures, fold hinges, and faults. The flow can be relatively continuous or more pulsed, as exemplified by porosity waves. The transport of dissolved constituents in the fluids occurs by advection (flow), diffusion, and mechanical dispersion, all of which are important during regional flow. Available models and field studies suggest that flow is dominantly upward in deep systems. Time-integrated fluid fluxes can vary by orders of magnitude. The smallest regional fluxes (<103 m3 m−2) are realized in dominantly pervasive flow systems where flow is distributed over a wide area and/or the availability of fluids is limited. Typical devolatilization of metasediments in orogenic belts is likely to produce larger fluxes ~103–104 m3 m−2 averaged over the regional scale. Still larger fluxes of ~104–106 m3 m−2 require channelization and focusing of flow into veins, shear zones, regional fracture systems, or other conduits. Fluxes of ~103 m3 m−2 or greater can produce substantial mass transfer and metasomatism by, for example, flow up or down temperature gradients or across lithologic contacts. Even when fluxes are smaller, the available flow and diffusion can drive mass transfer at more local scales. Despite several field examples, it remains unclear if large-scale heat transfer by fluids through the deep crust is a widespread phenomenon. Timescales of flow range from <103 years to the duration of orogeny. The shortest timescales correspond to flow associated with fracturing, orogenic thermal pulses, or other transient phenomena. The largest timescales reflect more progressive, long-term devolatilization of orogenic belts. A growing body of evidence indicates that significant flow can occur on geologically short timescales of less than ~106 years.