Physical activity is an effective intervention for alleviating many physical, emotional, and social stressors faced in survivorship, and for improving the quality of life of adults living with cancer. Social support is positively associated with physical activity and is important for helping adults living with cancer cope with the challenges of cancer and issues in survivorship. Group physical activity programs can provide opportunities for social support from other individuals living with cancer, as well as fitness professionals leading the program. The purpose of this thesis was to better understand what types of social support and supportive behaviours are perceived as supportive and are associated with greater physical activity and quality of life in group exercise oncology programs. The first study cross-sectionally examined the associations between 16 social support predictors and 5 quality of life outcomes, 5 self-report physical activity outcomes, and objectively measured average daily step count. Hypotheses were tested using multiple regression. Self-report moderate-to-vigorous physical activity was positively predicted by reassurance of worth support, general and social well-being were negatively predicted by thwarting relatedness, and physical well-being was positively predicted by social network within group exercise oncology programs. The second study examined current practices for training fitness professionals to provide and facilitate social support, experiences people living with cancer have of social support, and supportive behaviours and barriers for providing and obtaining support in online group exercise oncology programs. Training materials were reviewed, observations of fitness professional training and online exercise classes (N=10) were conducted, and adults living with cancer (N=19), and fitness professionals (N=15) were interviewed. Data analysis was guided by interpretive description methodology (Thorne, 2016), and interpreted in light of Feeney and Collins’ (2015) social support theory. Three themes were identified: creating a welcoming atmosphere, helping improve exercise ability and reach goals, and learning to provide and facilitate support online. Together these studies provide insight into specific behaviours adults living with cancer find supportive, and social barriers in the group exercise oncology context to inform fitness professional training and program development to enhance and promote a socially supportive environment.