In this thesis I report on investigations of the early socioemotional development of 4- month-old infants with Down syndrome (DS) in dyadic and triadic interactions, and comparisons of aspects of socioemotional behaviour of infants with DS with those of typically developing (TD) infants. The thesis incorporates theories relating to positive emotion, early infancy and Down syndrome research. I investigate 3 main debates: when compared to typically developing infants 1) infants with DS may have differences in emotionality, and perhaps social and emotional strengths 2) that infants with DS may have may different patterns of attention, perhaps preferring attending to a social partner over object 3) that the environment within which the infant with DS is raised is somehow different. I investigate these debates with consideration that all may relate to each other. I compare typically developing infants and infants with DS on measures of attention, emotion and sociocommunicative behaviour alongside measuring aspects of their environment. I analyse the behaviour of groups of infants in dyadic (TD: n=11, DS: n=10) and triadic social, and less social, situations (TD: n=10, DS: n=10). I investigate aspects of infant emotion and sociocommunicative behaviours and discuss how they may, or may not, be indicative of abilities in for example, social expectancy, person permanence and early joint attention. I also question whether different play partners relate to infant enjoyment. I report on data relating to infant temperament, and aspects of the very young infant’s environment (maternal caregiving preferences, optimism and demographics) in order to consider the relation of these variables to group and individual differences. I integrate findings from all studies and discuss individual differences. Key findings from the thesis were that DS infants were less fussy overall than TD infants, equally as positive and communicative, and demonstrated an ability to maintain longer social interactions compared to TD infants. DS infants had less interest in an object than TD infants, and more interest in the social partner. This was not due to infants with TD being unable to shift gaze, as infants from both groups shifted gaze comparably. TD infants were able to follow gaze to some extent, and demonstrated sensitivity to the timing and structure of the game of peekaboo. This suggests that at 4 months of age, TD infants may have some level of social expectancy regarding the rules and structure of social exchange, and have emerging joint attentional skills. DS infants did not follow gaze as successfully (although some did) and did not demonstrate such sensitivity to the timing and structure of the peekaboo game (although some did, and some TD infants did not) however enjoyed the game as much as TD infants. TD infant and mother pairs played the game of peekaboo differently to TD infant and experimenter pairs. In the main, infants with DS and mothers played similarly, and infants enjoyed the game as much, as DS-experimenter pairs; and this was comparable to how TD infants and experimenters played and enjoyed the game. No differences emerged between groups on measures of temperament, nor in relation to maternal factors such as parenting system preference or optimism. Neither did any of these measures relate to levels of infant positive emotion during positive, dyadic play. It was concluded that the ability for infants with DS to maintain prolonged social interactions with another at 4-months of age could be interpreted as a strength, perhaps due to an increased focus on the building blocks of later emerging social and emotional skills, that occur during face-to-face interaction. My thesis emphasises the importance in recognising strengths for those with DS, and acknowledging similarities rather than differences with the typical population. The value of this cannot be underestimated for those families (and educators, health professionals and carers) involved with DS. Recognition of sameness promotes an inclusive attitude to enable those with DS to integrate and to develop within a positive environmen