Rocks encode the sum of Earth processes that affected them during their ‘lifetimes,’ and most geologic research seeks to invert that information to refine our understanding of those processes. Metamorphic rocks record not just a single peak pressure–temperature (P–T) condition, cooling rate, assemblage, or texture, but rather have undergone evolving histories of changes in P and T, mineral abundances, rim compositions and textures, acting over the metamorphic lifespan of a rock, in response to heat flow, stress and strain, and inter- and intragranular movement of material. The greatest advances in understanding metamorphic rocks have recognized that metamorphism is a continuum and have presented data and models that directly address the continuum processes. Of the many approaches for investigating and interpreting metamorphic rocks, the characterization and quantitative modeling of geochemical zoning in metamorphic minerals plays a central role. Geochemical zoning is uniquely useful because it quasicontinuously records these metamorphic processes.