Tree shrews are small-bodied, scansorial, squirrel-like mammals that occupy a wide range of arboreal, semi-arboreal, and forest floor niches in Southeast Asia and adjacent islands. Comparative aspects of tree shrew biology have been the subject of extensive investigations during the past two decades. These studies were initiated in part because of the widely accepted belief that tupaiids are primitive primates, and, as such, might provide valuable insight into the evolutionary origin of complex patterns of primate behavior, locomotion, neurobiology, and reproduction. During the same period, there has been a renewed interest in the methodology of phylogenetic reconstruction and in the use of data from a variety of biological disciplines to test or formulate hypotheses of evolutionary relationships. In particular, interest in the comparative and systematic biology of mammals has focused on analysis of phy­ logenetic relationships among Primates and a search for their closest relatives. Assessment of the possible primate affinities of tree shrews has comprised an important part of these studies, and a considerable amount of dental, cranio­ skeletal, neuroanatomical, reproductive, developmental, and molecular evidence has been marshalled to either corroborate or refute hypotheses of a special tupaiid-primate relationship. These contrasting viewpoints have resulted from differing interpretations of the basic data, as well as alternative approaches to the evolutionary analysis of data.