Significant advances have been made recently towards understanding the properties of materials through theoretical approaches. These approaches are based either on first-principles quantum mechanical formulations or semi-empirical formulations, and have benefitted from increases in computational power. The advent of parallel computing has propelled the theoretical approaches to a new level of realism in modeling physical systems of interest. The theoretical methods and simulation techniques that are currently under development are certain to become powerful tools in understanding, exploring and predicting the properties of existing and novel materials. This volume from MRS brings together scientists from several subfields of materials theory and simulations to: make contact with traditional continuum approaches to materials theory; discuss critically current developments in computations and simulational approaches specifically aimed at addressing real materials problems, with an emphasis on parallel computing; and present examples of the most successful applications of computational and simulational work to date.