Chemical reactions at aqueous interfaces play major roles in Earth terrestrial and subsurface cycles of most reactive elements, trace elements, heavy/radioactive ions, and environmental pollutants. Surface Complexation Models appear to be one of the most successful models in geochemistry to predict the chemical equilibrium state of a given interface in contact with solution. They have provided the ability to understand, predict, and explain the sorption of protons, aqueous ions, and molecules to hydrated mineral surfaces using physical models for interfacial processes and energies. Volume 91A is intended to present the past, present, and future of Surface Complexation Modeling and nucleates the concepts for the next generation of more comprehensive SCMs that account for more complex chemistries and non-equilibrium phenomena.