ABSTRACT This chapter summarizes some important concepts in using molecular modeling methods for geochemical problems, with emphasis on chemistry as opposed to physics. Rather than summarize the most reliable ways of predicting crystal structures or physical properties and the equations of state of minerals, the chapter focuses on studies addressing problems associated with chemical reactivity. The development of a molecular-level understanding of low-temperature geochemical problems is theoretically challenging. Many of the most interesting questions are concerned with chemistry at the mineral-water interface ( Lasaga, 1990 ), and involve complex phenomena including hydrogen-bonding, the presence of an aqueous phase, minerals which may be spin polarized, and electron and proton transfer. In general, these systems are (at present) too computationally difficult for a purely first-principles approach. For example, only recently have ab initio electronic structure methods satisfactorily dealt with the binding energy and structure of the water dimer ( Feyereisen et al., 1996 ) 1 . Because of this computational complexity, it is often useful to couple theoretical and empirical modeling methods.