The Characterization of Biomolecular Interactions Using Fluorescence Fluctuation Techniques -- Characterization of Protein-Protein Interactions Using Atomic Force Microscopy -- Combined Solid-Phase Detection Techniques for Dissecting Multiprotein Interactions on Membranes -- Surface Plasmon Resonance Biosensing in the Study of Ternary Systems of Interacting Proteins -- Mass Spectrometry for Studying Protein Modifications and for Discovery of Protein Interactions -- H/2H Exchange Mass Spectrometry of Protein Complexes -- Elucidation of Protein-Protein and Protein-Ligand Interactions by NMR Spectroscopy -- Application of Isothermal Titration Calorimetry in Exploring the Extended Interface -- Solvent Mediated Protein-Protein Interactions -- Sedimentation Equilibrium Analytical Ultracentrifugation for Multicomponent Protein Interactions -- Structure Analysis of Macromolecular Complexes by Solution Small-Angle Scattering -- Fluorescence Detection of Proximity -- Steady-State and Time-Resolved Emission Anisotropy -- Analysis of Protein-DNA Equilibria by Native Gel Electrophoresis -- Electrospray Ionization Mass Spectrometry and the Study of Protein Complexes -- Sedimentation Velocity in the Study of Reversible Multiprotein Complexes. This comprehensive volume, written by experts in the field, brings together a coherent set of reviews of different biophysical methodologies for the study of reversible protein interactions. These reviews allow the reader to gain a deep understanding of the biophysical observables, to stimulate creative thought on how the techniques may be applied to study a particular biological system, and to foster collaboration and multi-disciplinary work. The side-by-side presentation of different approaches highlights aspects they have in common, as well as orthogonal viewpoints that may provide opportunities for synergistic combination. Protein Interactions is an ideal book for biochemists, protein scientists, bioenginners, and molecular and cellular biologists.