Hydrous phases are among the most important Earth components. They have technological and societal utility and are important for a broad suite of Earth processes, including the origin of life. From both thermodynamic and structural perspectives, however, they represent some of the most complex naturally occurring materials: their bonding often includes a combination of strong covalent, weak ionic, van der Waals, and hydrogen bonding, all within large unit cells. Most are solid solutions, and many are prone to variations in layer packing. Many prior studies of these materials have emphasized experimental measurements and analytic modeling of their thermodynamics. Such thermodynamic studies represent a fundamental tool for understanding present and past natural processes, including those that shaped—and continue to shape—the structure and evolution of our planet. Nevertheless, many properties of these materials and solid solutions are difficult to measure experimentally or model analytically. To make significant new progress and attain a deep and predictive understanding of these materials requires a more atomistic and theoretical approach.