This chapter focuses on protein-protein interactions, which are ubiquitous in the cell and are essential for signal transduction, metabolism and molecular transport. Protein-protein interactions occur, for example, between hormones and receptors to stimulate reactions to changes in other cells. Traditionally, protein-protein interactions have been determined individually via conventional wet-lab experimental methods such as immunoprecipitation and other pull-down techniques. Immunoprecipitation has been traditionally utilized to isolate and identify protein antigens that bind to specific antibodies out of a cell extract. Immunoprecipitation is a special case of a pull down experiment, where the target protein is an antibody. Protein arrays are basically an extension of traditional methods such as affinity chromatography and coimmunoprecipitation, and they provide an in-vitro alternative to the yeast two-hybrid system. Protein arrays are commonly applied to study the properties of receptor-ligand or enzyme-substrate compounds and can help to assess the binding capabilities of proteins to potential drugs.