l The interactions of protein molecules in solution dictate a number of practically important physical phenomena. The success of protein crystallogenesis, the conversion of individual protein molecules in solution to a macroscopic crystal suitable for diffraction studies, is ultimately determined by the initial protein aggregates formed. The equilibrium between protein molecules in solution and protein molecules in a crystal lattice, i.e. phase behavior, is governed by the pair potentials between the protein molecules. The ability of certain solutions (excipients) to stabilize protein solutions, i.e. to prevent or minimize aggregation in pharmaceutical formulations for extended periods of time (months), depends on the presence of net repulsive interactions between the protein molecules. A dilute solution thermodynamic parameter called the second virial coefficient has been proven to be an effective predictor for the above mentioned phenomena. This presentation will detail the conceptual aspects and utilization of this parameter for such applicationsater