The interactions of ions, organics, and microorganisms at the aqueous interface with kaolin-group minerals control many important geochemical processes in the environment. Kaolinite has both hydrophilic and relatively hydrophobic external surfaces that exhibit different adsorption phenomena. Our understanding of kaolin minerals in the environment is advancing as a result of molecular simulation and field studies. Molecular dynamics simulations reveal the structure and behavior of adsorbed ions and water molecules at the interface. The presence of microorganisms affects the formation and surfaces of kaolinite and halloysite. Mechanisms by which kaolin-group minerals complex, adsorb, and desorb radioactive pollutants in the subsurface can be understood by combining theory with observation.