Protein phase behavior is implicated in numerous aspects of downstream processing either by design, as in crystallization or precipitation processes, or as an undesired effect, such as aggregation. An improved understanding of protein phase behavior is therefore critical to developing rational design strategies for important process steps. This presentation will explore the phase behavior of monoclonal antibodies (mAbs), which may exhibit a wide range of forms despite the structural similarities at the molecular level. Liquid-liquid separation, aggregation, gel networks, and crystals have been observed. A systematic study of numerous factors, including the effects of solution composition and pH, is conducted in order to explore antibody behavior. Examples of phenomena observed for individual mAbs include a significant dependence of the spinodal boundary on the cation in sulfate salts and non-monotonic trends in pH dependence. Protein-protein interactions are used to interpret these observations. Comparisons of behavior among different mAbs, and possible mechanistic origins, will be discussed.