Most animal groups contain individuals of just one species, but some live in mixed-species groups. In order to study this there must be clear criteria for determining what constitutes a mixed group. However, providing a standard definition is problematic due to variations in environment/habitat, duration of associations and stability between different taxa. In primatological studies, two or more species are considered to be in a group when individuals of different species are within a certain distance; ≤ 20m or ≤ 50m in the wild, and ≤ 50cm in captivity. Mixed-species groups are commonly reported amongst Neotropical primates, with the most frequently documented between tufted capuchins (Sapajus sp) and squirrel monkeys (Saimiri sp). For this reason, two mixed-species groups of tufted capuchins and squirrel monkeys were housed at the Living Links, RZSS, Edinburgh Zoo. An important contribution of my thesis will be to explore what criteria can be used in determining what constitutes a ‘group’ beyond arbitrary distances, based on social network analysis, behavioural synchrony and evaluating space use. Data will be presented on the two mixed-groups at Living Links, and the wild troops encountered in the Raleighvallen Nature Reserve, Suriname.