The area of animal counting has historically been the subject of a long and colorful debate, but only more recently have systematic, more rigorous experimental efforts to evaluate numerical abilities in animals been undertaken. This volume contains chapters from investigators in a range of disciplines with interests in comparative cognition. The studies described characterize the emergence of number-related abilities in rats, pigeons, chimpanzees, and humans, bringing together — for the first time in one volume — the rich diversity of cognitive capabilities demonstrated throughout many species. The data and theoretical perspectives shared will likely serve to provoke much thought and discussion among comparative psychologists and fuel new research and interest in the field of animal cognition.