The display of diverse repertoires of proteins on the surface of filamentous bacteriophage offeres a new way of selecting proteins with predefined specificities. The affinity selection process can be carried out due to unique feature of the phage expression system, the direct physical linkage between genotype and phenotype of displayed proteins. Phage dysplayed libraries of proteins have been sucessfully used in a number of applications, including selection of monoclonal antibodies, receptor ligands and enzyme inhibitors, cDNA expression, epitope mapping, studies on protein?protein and protein?nucleic acids interactions. The phage dislay system is also a very decent tool to perform in vitro evolution of protein properties. This approach is based on construction and screening of reprtoires of mutagenized proteins or protein domains and affinity selection of molecules with most desirable features.