Microbial infections are dependent on the panoply of interactions between pathogen and host and identifying the molecular basis of such interactions is necessary to understand and control infection. Phage display is a simple functional genomic methodology for screening and identifying protein-ligand interactions and is widely used in epitope mapping, antibody engineering and screening for receptor agonists or antagonists. Phage display is also used widely in various forms, including the use of fragment libraries of whole microbial genomes, to identify peptide-ligand and protein-ligand interactions that are of importance in infection. In particular, this technique has proved successful in identifying microbial adhesins that are vital for colonization.