This volume represents a comprehensive examination of the newly recognized callimico/marmoset clade, which includes the smallest anthropoid primates on earth. It will explore these diminutive primates in their entirety, with sections on phylogeny, taxonomy and functional anatomy, behavioral ecology, reproductive physiology, as well as address critical conservation issues and the need for conservation action. The topics specifically selected for this volume are pivotal for understanding the evolutionary adaptations and divergence of any primate group, and especially one as diverse and curious as this. The discoveries of new taxa over the last fifteen years along with new genetic data have transformed this group from three genera (one with only a distant relationship to the others) and five recognized species, to five closely related genera, comprising at least 22 species. This volume will be the first to synthesize data on these newly recognized taxa.This volume is an international endeavor, bringing together primary callimico and marmoset researchers from around the globe, including Brazil and the United States as well as Greece, Italy, Switzerland, and Germany. One of the merits of this volume is that it will serve as a readily accessible work that includes the major findings of several key international researchers whose work has not been easily available to English-speaking scholars. In addition, it draws together lab and field researchers, geneticists, anatomists, and behaviorists in an integrated volume that will provide the most detailed and thorough work on either callimicos or marmosets to date. This volume will also provide a timely forum for identifying future avenues of action necessary for more fully understanding and protecting this intriguing primate radiation.