People with primary brain tumour typically experience high levels of functional impairment with an uncertain and often poor prognosis. Outcome studies highlight that up to 48% of people with brain tumour experience clinical levels of psychological distress that can persist throughout the course of living with the illness. Strong social ties can protect against the adverse effects of chronic illness on psychological wellbeing; however, the functional impairments associated with brain tumour can affect relationships and reduce social participation. There is currently poor understanding of the impact of primary brain tumour on social participation, and how social networks influence wellbeing for this population. Comprising four studies, the broad objective of this thesis was to advance understanding of the impact of brain tumour on social groups and the implications for psychological wellbeing. Study 1 employed a cross-sectional design to investigate how social group memberships influence the relationship between subjective functional impairment and psychological wellbeing. Seventy adults (60% female; aged 22-75 years) with primary brain tumour (46% benign; 18% low-grade; 36% high-grade) undertook a telephone-based assessment including self-report measures of cognitive and physical impairment, social group memberships, confidence in support, and psychological wellbeing. Higher functional impairment was associated with a loss of pre-existing social groups which, in turn, was associated with greater depression and anxiety symptoms. Further, confidence in social groups moderated the relationship between functional impairment and psychological wellbeing. Specifically, those perceiving high levels of functional impairment who had high confidence in their social groups reported lower depression and anxiety than those who had low confidence in their social groups. Study 2 involved a meta-synthesis that aimed to identify, appraise and integrate the findings of qualitative studies that reveal the impact of brain tumour on social networks. The findings of 21 eligible studies were synthesised to reveal three core themes depicting the social trajectory of living with brain tumour: 1) Life disrupted; 2) Navigating the new reality of life; and 3) Social survivorship versus separation. The findings of the meta-synthesis highlighted that changes to social participation commonly occurred throughout the illness including a loss of pre-existing social networks and the emergence of new ones. This review identified a need for further qualitative research to understand how brain tumour affects peoples’ ability to maintain and/or rebuild social networks, and how social networks influence psychological wellbeing. Studies 3 and 4 employed a phenomenological qualitative methodology to explore the lived experience of the social and psychological impacts of brain tumour. Twenty participants (65% female; aged 22-69 years) were purposively selected from the Study 1 sample (N = 70) according to the following characteristics: type and grade of brain tumour (i.e., benign, low grade glioma, malignant/high grade glioma); sex; age (<40, 40-55, >55 years) and time since diagnosis (≤ 24 months or >24 months). Two interviews, three months apart, comprised a Social Identity Mapping exercise (Cruwys et al., 2016) and semi-structured interview exploring changes in social groups and the impact of social groups on psychological wellbeing. Study 3 aimed to understand how brain tumour influences peoples’ ability to manage, maintain, and rebuild their social networks. The main research questions were: 1) how does brain tumour affect people’s ability to stay connected to their social groups? and 2) how do they manage to stay connected or build new connections despite their functional impairments? Two overarching and interrelated themes emerged: engaging and connecting and then versus now. Many individuals experienced significant barriers to social participation including functional impairments, fear of negative evaluation and attributions about the discomfort of others. The ability to harness facilitators and/or develop strategies to overcome these barriers influenced whether individuals experienced stability, maintenance and expansion, loss and rebuilding, or loss and shrinkage of their social networks over time. Study 4 aimed to explore the meaning and functions of social groups, and how these influence wellbeing after brain tumour. The main research questions were: 1) what are the meaning and functions of social groups after brain tumour? and 2) how do individuals perceive that social groups influence their wellbeing? Two major themes emerged relating to the ways in which social groups contribute to and/or detract from wellbeing. The first theme, feeling understood and fulfilled by social groups, depicted perceptions of social groups as positively contributing to wellbeing and sense of identity. The second theme, feeling disappointed in, devalued or drained by social groups, highlighted ways in which social groups detract from wellbeing. Overall, this thesis advances understanding of how primary brain tumour affects people’s ability to stay connected to their social networks and how social groups influence wellbeing. Specifically, Study 1 highlighted both mediating and moderating pathways through which social groups can influence the relationship between functional impairments and psychological wellbeing. Study 2 yielded a novel framework depicting the changes in social roles and relationships across the trajectory of living with brain tumour. Study 3 highlighted that an interplay of barriers, facilitators and strategies influence people’s ability to manage, maintain and/or rebuild social networks after brain tumour. Study 4 revealed that social networks can both contribute to and detract from psychological wellbeing after brain tumour. Collectively, these findings highlight the need for supportive care interventions to focus on enhancing social participation as an avenue for improving psychological wellbeing after brain tumour.