Mixed-species groups, which represent a special mode of ā€œgroup-livingā€ have been described in a number of taxa, however there is no consensus as to how these groups are defined and measured: (1) some authors refer to a collective noun (e.g. ā€˜groupā€™, ā€˜flockā€™, ā€˜troopā€™, ā€˜aggregationā€™) without explicitly defining the term; (2) some authors provide general definitions based on behaviour and interactions that are not explicitly measured and 3) some provide quantifiable definitions based on distances, but the distances appear arbitrary. There are four prerequisites that constitute ā€œgroupnessā€: (1) group stability; (2) an element of socialness; (3) proximity and behavioural coordination in time and space and; (4) a minimum number ā‰„2. I review the literature on single-species and mixed-species animal groups, focussing on the costs and benefits of groups in relation to the two main selective forces that drive grouping and ultimately lead to improved fitness in animals: foraging advantages and reduced risk of predation. This thesis uses a multi-methodological approach of field and captive observations, in order to quantify the ā€˜groupnessā€™, of mixed-species groups of Guianan brown capuchins (Sapajus apella) and Guianan squirrel monkeys (Saimiri sciureus), based on the four aforementioned prerequisites for grouping. My fieldwork in Suriname, South America, at two study sites showed extensive variability but overall, the species were rarely in proximity. I collected behavioural data on two mixed-species groups of Sapajus and Saimiri at the Living Links to Human Evolution Research Centre, RZSS, Edinburgh Zoo. I investigated the groupness of the monkeys in both single-species and mixed-species groups, using different methodological approaches to quantify the different prerequisites of grouping. Prerequisite (3) was examined via 3D space use in enclosures (proximity in time and space) and group behavioural synchrony (coordination in time and space). Prerequisites (1) and (2) were examined using social network analysis. Sapajus and Saimiri occupied different spaces in their shared enclosures, their behaviours were largely asynchronous and social networks produced two distinct species clusters. I conclude that Sapajus and Saimiri were not behaving as true mixed-species groups, and that there is fluidity in patterns. In addition to the theoretical interest in understanding mixed-species groups, this thesis explores how the findings relate more generally to welfare in captivity and provides operational definitions that distinguish mixed-species groups from mixed-species associations and aggregations.