Uncovering the functions of thousands of gene products, in various states of post-translational modification, is a key challenge in the post-genome era. To identify small-molecule probes for each protein function, high-throughput methods for ligand discovery are needed. In recent years, small-molecule microarrays (SMMs) have emerged as high-throughput and miniaturized screening tools for discovering protein-small-molecule interactions. Microarrays of small molecules from a variety of sources, including FDA-approved drugs, natural products and products of combinatorial chemistry and diversity-oriented synthesis, have been prepared and screened by several laboratories, leading to several newly discovered protein-ligand pairs.