For Non-Governmental Organisations (NGOs), social impact is becoming more important in their organisational activities. Although social impact has gained more attention in recent years, social impact remains a term with a lack of clear definition, causing divisions on how to measure it correctly. This thesis aims to contribute to a clearer perception of social impact and its assessment. NGOs are critical actors in achieving the 17 Sustainable Development Goals (SDGs), with social impact being an integral part of all 169 SDG targets. The study is a multiple case study of social impact that utilises an inductive qualitative approach to analyse NGOs that are part of the Blue Communities (BCs) program of Plastic Oceans International. The study aims to research the extent of social impact achieved by the BCs, how they assess their social impact, and whether there is movement for policy influencing. Interviews were conducted with participants from various BCs worldwide to gather data and answer the research question. The research concluded that for the BCs, social impact is translated as positive changes in people's behaviours and mindsets. Social impact is complex to measure, which makes it challenging for the BCs to have a standard social impact measurement method. However, even though challenges exist regarding social impact, the BCs are achieving social impact, especially at alocal level, and efforts can be observed that could lead to future policy influencing activities. The thesis contributes to the research gap regarding defining social impact and how to measure it.