Executive Summary Objectives: Co-creation is a promising avenue for solutions to public health issues. To date, coherent, rigorous and systematic frameworks for co-creation, including theoretical and methodological principles, are currently lacking. Health CASCADE aims to develop such a framework for co-creation in public health. This report describes the first steps towards developing theoretical principles for co-creation in Public Health including: proposing a definition and exploring a potential theoretical lens for understanding evidence-based co-creation. There are wide-ranging definitions given for the term ‘co-creation’ but in short, Health CASCADE tentatively uses the following working definition: “An evidence-based methodology for the development, implementation and evaluation of innovations through continuous, open collaboration, interactional knowledge production and shared decision-making among key stakeholders, directed at improving public health”. Methods: A (traditional) literature review and primary semi-structured interview (undertaken with researchers in the field of co-creation within public health) were the twofold methods adopted. Key Findings: Health CASCADE is seeking to provide a methodology for the combination of an evidence-based approach (“mode 1”) and co-creation (”mode 2”). Further, participatory typologies, like Arnstein’s ladder, are very simplified conceptualisations of how participation (adult and child respectively) is seen in a project / research / programme, and so caution must be exercised. It became clear that co-creation / transdisciplinary knowledge claims are not referenced to a solely objective or subjective reality. Further when exploring suitable lenses, an ontology called ‘critical realism’ emerged in the literature and the primary interviews, as a potentially suitable lens which may be worthwhile to investigate further. For instance, as the propositions of critical realism considers objective reality as well as the role of subjective interpretation in framing reality, it could provide the balance needed for allowing co-creation. The application of thematic analysis to the primary interview data revealed some important insights. For example, when incorporating paradigms within co-creation theory framework, the interviewees advocated for the best principles from multiple paradigms to be used, and thus not merely relying on one paradigm. This report was produced as a deliverable (D1.1) for December, 2021.