A collection of highly successful biochemical, biophysical, genetic, and computational techniques for studying protein–protein interactions. These readily reproducible methods demonstrate how to identify protein interaction partners, qualitatively or quantitatively measure protein-protein interactions in vitro or in vivo, monitor protein–protein interactions as they occur in living cells, and determine interaction interfaces. The techniques described utilize a variety of cutting-edge technologies, including surface plasmon resonance (SRP), fluorescence resonance energy transfer (FRET), fluorescence polarization (FP), isothermal titration calorimetry (ITC), circular dichroism (CD), protein fragment complementation assays (PCA), various two-hybrid systems, and proteomics and bioinformatics-based approaches, such as the Scansite program for computational analysis. These powerful tools are essential for deciphering how proteins interact with each other to form biological networks, as well as for unraveling protein-protein interactions in disease to find novel therapeutic targets.