Over the past decade, several methods have been developed for the construction of DNA-encoded peptide libraries. The common principle behind all these methods is the establishment of a physical linkage between a displayed peptide and its encoding DNA. Vast libraries can be generated, binding peptides can be isolated with simple selections, and the sequences of selected peptides can be rapidly determined from the sequence of the linked DNA. As a result, DNAencoded libraries can provide specific ligands for essentially any protein. These ligands can be used to determine the natural binding specificities of protein–protein interactions, and this information can be used to identify natural binding partners or to aid the design of organic mimics. Binding peptides can also be used for target validation and the development of high-throughput screens for small-molecule libraries. Finally, binding peptides themselves could prove useful as drugs.