This first volume in the series on "Comparative Brain Research in Mammals" deals with the brains of Insectivora. The importance of Insectivora lies above all in the fact that, (1) insectivore-like ancestors are regarded as the initial group for the evolution of higher mammals, and (2) the insectivore brains retained many conservative traits, though the animals have adapted to different living environments. Therefore, the study of Insectivora brains enables an insight into the size and composition of the brain structure of earlier ancestors of the higher mammals including primates and humans; in addition, it illumi nates the various evolutionary trends which made the successful adaptation to different biotopes possible. The alterations which the brain has experienced in the course of the phylo geny and the related adaptive radiation will be examined in the succeeding volumes on the brains of other Eutheria, e.g., bats (Volume 2) and primates (Volume 4). In Volume 3 the brains of marsupials will be compared with those of conservative and evolved Eutheria.