The intricate surface topology and charge distribution on protein molecules give rise to a rich spectrum of inter-molecular interactions. Hence, structure and sequence govern how proteins interact with other proteins, surfaces and small molecules and lead to exiting mechanisms such as electrostatic steering, charge regulation and depletion interactions. It has recently been shown experimentally that aqueous mixtures of small, oppositely charged proteins -- lysozyme and alpha-lactalbumin, for example -- assemble to form thermodynamically stable micro-spheres in a highly salt and temperature dependent manner. The question is what happens at the molecular level? Using Monte Carlo computer simulations we have shown that the pair-wise interaction between the before mentioned proteins is dominated by electrostatics and that the protein-protein interaction is highly anisotropic, with protein alignments of up to 80 percent! Molecular simulation studies of merely two proteins is, however, not sufficient to predict phase transitions as observed experimentally. For this we need to increase the number of protein molecules and study the solution under constant pressure. Preliminary results of such formidable simulations, besides the phase diagram, lets us retrieve the full microscopic picture of the many-body protein-protein association process. Specifically we investigate the following: * The phase behavior of a variety of mixed protein solutions at various solution conditions such as pH, salt concentration, molar ratio etc. * A molecular level explanation for the formation of protein micro-spheres. * Develop reliable coarse grained simulation techniques for predicting thermodynamic properties - osmotic second virial coefficients, structure.