The challenge of crystallizing proteins has led to a significant amount of research in understanding protein self-association and assembly. Arguably the most influential finding in this field in the past decade has been that weakly attractive protein interactions, characterized in terms of the osmotic second virial coefficient, correlate with solution conditions that are conducive to crystallization. Recent work in this area has focused on the development of more efficient techniques for measuring the second virial coefficient, as traditional characterization methods suffer from poor efficiency in terms of time and protein consumption. The resulting measurements have provided new insights into patterns of protein interactions and their relation to protein phase behavior.