The purpose of this research is to explore the sustainability of twelve Social Enterprises (SEs) and examine the role marketing plays in their long-term sustainability. It has focused upon day-care centres which provide work-based training and skills development for adults with learning, mental and physical disabilities. A case study methodology using semi-structured interviews, observation and document analysis was used, and through a grounded theory approach, data was analysed through NVivo. The findings indicate marketing being utilised by SEs, but for the majority, how the Managers understood and defined marketing was based upon a goods-dominant approach, which is argued to be inappropriate for these types of service organisations. This thesis has advocated that is only possible for SEs to be sustainable in the long-term when they view themselves as service organisations rather than through a goods-dominant lens. Thus, this research has identified the relevance of a services-marketing approach for SEs delivering public services. Drawing upon literature from services and relationship marketing and rooted within the public services-dominant logic this thesis has reconceptualised marketing for SEs. It has demonstrated the significance of services theory in understanding SE and by developing the new model of marketing, it has shown the significance of services theory in understanding the tension in their double bottom line. It has identified co-creating value with multiple stakeholder groups as a key route to long-term sustainability.