The papers in this issue stem from a symposium presented at the 1992 International Primatological Congress in Strasbourg, France. Due to the paucity of information on social organization, reproduction, and the relation of these to ecology in neotropical primates, we organized this symposium, which included researchers conducting comparative studies and those examining aspects of socioecology in single platyrrhine species. The papers in the symposium are organized around three issues: (1) the relation between body size, ecology, and reproduction (papers by Dietz et al., Tardif, and Williams et al.); (2) ranging and dispersal patterns as determinants of social structure (papers by Boinski and Mitchell, Norconk and Kinzey, and Kinzey and Cunningham); (3) phylogenetic and comparative analyses of behavioral and anatomical traits (Ford and Garber). Within these areas, the papers provide new information and new syntheses of existing information on platyrrhine socioecology. In addition, many detail innovative methodological approaches which could fruitfully be applied to other primates. © 1994 Wiley-Liss, Inc.