Publisher Summary Protein–protein interactions play crucial roles in a wide variety of processes, which define cell structure and function. This chapter describes the emerging tools and strategies for determining the kinetics and thermodynamics of protein–protein interactions. These biophysical methods can be used to play dominant roles as core tools for studying the structural and functional molecular properties of protein–protein interactions. In general, these methods require relatively small amounts (milligrams) of protein material and are universally applicable to the study of proteins. Although biophysical methods are capable of providing molecular-level details of protein–protein interactions, the increase in resolution to the molecular level is accompanied by an increased need to validate interpretation of the data. The chapter discusses the synergistic benefits of using multiple independent methods.