The new sixth edition of this classic work continues the longstanding tradition of excellence established originally by Ernest P. Walker and his collaborators 35 years ago. This edition is about 25% larger than the previous one, and it covers all genera of extant mammals, as well as genera that have become extinct during the past 5000 years, including the thylacine and woolly mammoth. The vast majority of generic entries have been updated, and more than 2700 new references have been added. The volumes continue to be extremely readable for both the scientific community and the general public, and most genera are documented photographically in more than 1700 illustrations that accompany the text. A new brief introduction to the class Mammalia discusses some of the agreements and controversies concerning the origins and higher taxon relationships among mammalian orders, with a reasonable summary of literature through 1993-1994. This, however, is not a major focus of the books, and the strength of this work continues to be the readable accounts of the physical traits, habitat, locomotion, activity patterns, range, reproduction, population dynamics and social life, and endangered status of all extant mammalian genera. As with the previous editions of this classic work, this two-volume set remains the classic reference work on the biology of mammals, and Ronald Nowak is to be commended for continuing the high standard of excellence initiated by Ernest Walker.