Fluids are ubiquitous in Earth's lithosphere. Reactions between fluids and rocks critically impact petrophysical rock properties and the geochemistry and geodynamics of the Earth at all scales. By enhancing reaction kinetics and mass transport, fluids play a fundamental role in geological processes from the Earth's surface down to the deep lithosphere. Lithospheric deformation in all tectonic settings is strongly influenced by fluids, which contribute to strain localization, earthquake nucleation, as well as potential rheological weakening or strengthening triggered by advective fluid flow and metasomatism. Multiple and complex feedbacks between host rock and percolating fluids are known to lead to the formation of economic mineral deposits in a great variety of tectonic settings from the seafloor to the roots of mountain belts. Fluids can also provide a favorable environment for life development in unexpected settings.