Peptide libraries may be constructed by grafting, in vitro, random DNA sequences into a carrier gene and then introducing the degenerate hybrid coding sequence into an expression organism. This review will focus on phage display, which was the first expression organism for peptide library expression to be described and which still maintains predominance in this area because of its simplicity, minimal cost, ease of manipulation, power and robustness. Using phage as the host, a repertoire of random peptides can be expressed that may be searched by a variety of screening or selection procedures. By physically associating each element of the peptide library with its coding sequence, selection for a property of a specific peptide results in the enrichment of the corresponding gene thus facilitating its cloning and amplification. This review focuses on the construction and screening of peptide libraries displayed on filamentous phage capsid and only briefly discusses the display of proteins and protein domains.