Gene insertion into the avian germ line is reviewed. Useful pathogen-derived and animal-derived resistance genes that could be inserted into the avian germ line to convey resistance to pathogens are discussed. Methods of germ line insertion that have been successful in mice are summarized and their possible application in future avian germ line research are described. The successful transfer of retroviral genes into the chicken germ line using replication-competent recombinant retroviruses is summarized and several potentially useful transgenic chicken lines are described. The attempts to use one-round replication-defective retroviral vectors have been complicated by the detection of replication-competent retrovirus in chicks from injected eggs.