Publisher Summary This chapter discusses germ-free (gf) animal techniques and their applications. The development of a gf vertebrate animal as a generally useful experimental system has attained a suitable stage of fruition. The methodology of gf birds and mammals, their characteristics, and some of their applications are described in the chapter. Facilities for using gf animals can be relatively simple and inexpensive. However, long-term rearing and the development of breeding colonies of gf mammals and birds require more extensive operations. More such colonies are needed if full advantages of this technique are to be realized. Colonies of gf animals of specified genetic strains should be established for the common laboratory species. The development of devices for shipping gf animals will be needed in connection with the facilities.