This thesis sought to investigate objectification theory (Fredrickson & Roberts, 1997) in the context of Social Network Sites (SNSs). Mixed methods were employed to investigate whether SNSs cultivate objectifying environments and contribute to objectification, and to explore the consequences of SNS use and objectification for psychological functioning. A quantitative self-report study assessed the relationship between SNS use, selfsurveillance (the behavioural component of self-objectification), surveillance of others, well-being and mood state depression. Using a retrospective baseline measure and nine subsequent ecological momentary assessments completed over a week, trait and state variables were investigated with 193 females and 49 males ranging in age from 18-68 years. Online social comparison and body shame strongly predicted state and trait selfsurveillance and surveillance of others. SNS behaviours including photo-based behaviours, also predicted state self-surveillance. SNS variables including passive SNS use and objectification variables including self-surveillance and body shame predicted trait wellbeing. Time spent on SNSs and body shame predicted state depression. Focus groups with 32 emerging adults (14 female and 18 male) explored whether participants perceived SNSs as objectifying environments, and whether SNSs impacted on their psychological functioning and perceptions of their own and others’ appearance. Using thematic analysis four themes were identified; “SNS use, body image and gender dynamics”, “SNS use and psychological functioning”, “SNS use and the cultivation of an ideal self” and “Pervasiveness of SNSs”. Results indicated SNSs were environments in which objectification occurs and in particular online social comparison was identified as contributing to objectification and negative psychological functioning. Collectively the findings suggest objectification occurs and is facilitated by SNSs, and SNS use and self-objectification have consequences for psychological functioning. In particular, this thesis provides strong support for the role of online social comparison and body shame in contributing to objectification and negative psychological functioning.