Public and patient involvement (PPI) has been identified as an increasingly desired and, often, required component of trial methodology–leading to higher quality, more accessible and relevant clinical research, alongside increased recruitment, funding success and insight into research impact. However, despite the great variety of frameworks and checklists available for assessing PPI, most are limited with respect to important features (e.g. applicable in specific contexts only, fail to clarify what should be assessed and reported, lack the necessary comprehensiveness or are biased in favour of researcher reporting). Thus, the current research aimed to address such limitations through the development of a new checklist, the EPPIIC, through review, thematic analysis and ‘meta-evaluation’ in conjunction with PPI engagement. Upon completion of the EPPIIC, three thematic ‘sub-scales’ emerged: (1) Policy & Practice, (2) Participatory Culture and (3) Influence & Impact. All findings are presented and discussed in light of theory and research. Notably, findings recommend EPPIIC as a useful means of assessing PPI in future trials.