Protein microarrays, also known as protein chips that provides a versatile platform for characterization of hundreds of thousands of proteins in a highly parallel and high-throughput way. It is an important class of proteomic technologies that are in fast becoming crucial tools in biochemistry and molecular biology. Two major classes of protein microarrays are defined to describe their applications: analytical and functional protein microarrays. Analytical protein microarrays, mostly antibody microarrays, have become one of the most powerful multiplexed detection technologies. Functional protein microarrays are being increasingly applied to many areas of biological discovery, including studies of protein interaction, biochemical activity, and immune responses.