The evolution of the Earth's lithosphere is affected in a major way by metamorphic processes. Metamorphism affects the lithosphere's chemical and mineralogical composition, as well as its physical properties on scales ranging from a nanometer to the size of tectonic plates. Studies of metamorphism during the last couple of decades have revealed that fluids are as important in a changing lithosphere as water is in the biosphere. History-dependent characteristics of metamorphic rocks, such as their microstructure, compositional variation, and deformation features, reflect the dynamics of fluid–rock interactions. Migration of the fluids produced during prograde metamorphic processes or consumed during retrogression links metamorphism at depth to the evolution of the hydrosphere, the atmosphere, and the biosphere.