The title The mentalities of gorillas and orangutans: Comparative perspectives was inspired by Köhler's famous book The mentality of apes (1927), in which apes were implicitly equated with chimpanzees. This book focuses on two other great apes that were less known in Köhler's day. It is the fourth in a related series of edited volumes on cognition of great apes. The first volume, “Language” and intelligence in monkeys and apes (Parker & Gibson, 1990), emphasized the importance of using models from developmental psychology to compare the cognitive and symbolic abilities of monkeys, apes, and humans. The second volume, Self-awareness in animals and humans (Parker, Mitchell, & Boccia, 1994), used developmental frameworks to compare manifestations of self-awareness in monkeys, apes, and humans. It was aimed at broadening the scope of research in this subject to extend beyond the classical mark test for mirror self-recognition. The third volume, Reaching into thought (Russon, Bard, and Parker, 1996), focused on cognitive abilities of great apes using models from developmental psychology, but extended the scope to include studies in wild populations. This most recent volume continues the tradition of using models from developmental psychology to compare the cognitive abilities of great apes, and that of including studies from both captive and wild populations. It also expands the preview to include studies of taxonomy and phylogeny (Begun, this volume) and the brain (Semendeferi, this volume). It differs from the other volumes in focusing primarily on gorillas and orangutans. The aim of this volume is to redress the chimpocentric imbalance in attention to chimpanzees and bonobos at the expense of the other great apes.