The filamentous bacteriophage M13 can be used to display foreign peptides and proteins, allowing the construction of therapeutic, diagnostic and technological tools of broad utility. Foreign peptides or proteins have been inserted at or near the amino termini of the viral structural proteins gp3 and gp8. Both of these sites appear to have intrinsic limitations that need to be better understood for the technology to reach its full potential. Recent analyses of the normal functions of these proteins provide a basis for understanding these limitations and may allow the identification of other potential sites for insertions. Conversely, detailed analysis of the nature of viable and lethal insertions is providing information about the structure and assembly of the phage not obtainable by other means.