Environmental sustainability has become an increasingly relevant concern for non-profit sport organizations (NPSOs), despite their primary focus on sport participation. This thesis examines how environmental sustainability is perceived, expected, and negotiated at the level of individual club members and sport federations. A sequential mixed-methods design combines a survey of 1,751 non-profit sport club members with a multiple-case study of ten European sport federations. At the member level, findings show that members ascribe environmental responsibility to their club and evaluate its environmental engagement against these expectations. When perceived environmental sustainability falls below expectations, perceived value and satisfaction are lower, with implications for loyalty. At the federation level, environmental sustainability adds a further layer of institutional complexity alongside Sport-for-All, High-Performance, and Professionalization logics. The findings identify different pathways through which environmental sustainability-related tensions become salient and show that their navigation depends on context-specific dynamic capabilities. Overall, the thesis highlights environmental sustainability as an emerging organizational concern requiring continuous adaptation to stakeholder expectations and persistent institutional tensions.