The close coordination of family groups of siamang (Symphalangus syndactylus) by subtle visual signals contrasts with the very loud and complex calls which figure prominently in relations between groups to space them effectively through the habitat. The siamangs' repertoire of facial expressions, vocalisations, and expressive gestures, postures and movements are contrasted with that of the white-handed gibbon, Hylobates lar. The ways in which siamang move about their small range in a highly coordinated manner, with frequent pauses for resting and grooming, and for feeding, are described. Reference is made to the structure of group calling bouts and to conflicts between groups. There is discussion of the possibility that the group cohesion results from group members becoming able to predict accurately the movements of each other, and of the possible way in which aggressive acts are countered by conciliatory ones.