Phage display is achieved by fusing polypeptide libraries to phage coat proteins. The resulting phage particles display the polypeptides on their surfaces and they also contain the encoding DNA. Library members with particular functions can be isolated with simple selections and polypeptide sequences can be decoded from the encapsulated DNA. The technology's success depends on the efficiency with which polypeptides can be displayed on the phage surface, and significant progress has been made in engineering M13 bacteriophage coat proteins as improved phage display platforms. Functional display has been achieved with all five M13 coat proteins, with both N- and C-terminal fusions. Also, coat protein mutants have been designed and selected to improve the efficiency of heterologous protein display, and in the extreme case, completely artificial coat proteins have been evolved specifically as display platforms. These studies demonstrate that the M13 phage coat is extremely malleable, and this property can be used to engineer the phage particle specifically for phage display. These improvements expand the utility of phage display as a powerful tool in modern biotechnology.