The notion of customers co-creating value with the firm has recently gained considerable attention within the service marketing discipline. The Service-Dominant (S-D) and the Customer-Dominant (C-D) logic in marketing in particular emphasise the active role of customers in the co-creation of value. But further theoretical insights are needed into the process of value co-creation. Specifically, customer-to-customer (C2C) value co-creation that takes place as customers come together to socialise, interact with each other and to be co-present in socially dense service settings, requires further conceptualisation. C2C value co-creation is explored in this thesis in the socially dense service setting of multi-day outdoor festivals, using the concept of value-forming social practices as a theoretical lens. The methodological design is guided by the social constructionist stance, which complements the practice-based value approach in co-creation research by emphasising the importance of social contexts. Methods adopted include ethnographic-style participant observation, document and visual materials analysis, and a total of 52 in-depth interviews at five different UK-based outdoor multi-day festivals. Interpretive analysis identifies six distinctive C2C co-creation practices: Belonging, Bonding, Detaching, Communing, Connecting and Amiability. Each practice is described in terms of the actions in which it is embodied. The practices are positioned in a two-dimensional framework, with the Value orientation and the Value immersion dimensions reflecting the complexities and ambiguities that exist in social contexts. Aspects of subject- and situation-specific practice elements are examined with regard to their role in influencing the C2C co-creation process at festivals. Practice-based segmentation and social servicescape design strategies are proposed, which can be used to support and facilitate C2C co-creation. A theoretical contribution is made to the body of knowledge in service marketing, and the S-D and C-D logics in particular, by advancing understanding of what specifically is involved in C2C value co-creation. The thesis also offers holistic insights relevant for service marketing practice. It provides tangible recommendations that could lead to more favourable social outcomes for customers and consequently, competitive advantage for the firm