A peptide library is a collection of peptides that are synthesized or derived by chemical or physical modification. Synonyms for peptide library include peptide pool, peptide mix, peptide diversity set, and peptide repertoire. The diversity of these libraries can range from a few hundred to millions of peptides. Peptide libraries are useful for studying protein-ligand interactions, identifying peptide ligands of specific receptors, and developing new drugs. The use of peptide libraries has expanded greatly in recent years due to the development of high-throughput screening technology and modern computational and analytical methods. With its numerous synonyms, the peptide library has become an invaluable tool in the exploration of biochemical and therapeutic applications.