BACKGROUND: Patient and public involvement (PPI) in clinical trials aims to ensure that research is carried out collaboratively with patients and/or members of the public. However, current guidance on involving clinical trial participants in PPI activities is not consistent. METHODS: We reviewed the concept of participant involvement, based on our experience. Two workshops were held at the MRCCTU at UCL with the aim of defining participant involvement, considering its rationale; benefits and challenges; and identifying appropriate models for participant involvement in clinical trials. We considered how participant involvement might complement the involvement of other public contributors. Both workshops were attended by two patient representatives and seven staff members with experience of PPI in trials. Two of the staff members had also been involved in studies that had actively involved participants. They shared details of that work to inform discussions. RESULTS: We defined trial participants as individuals taking part in the study in question, including those who had already completed their trial treatment and/or follow-up. Because of their direct experience, involving participants may offer advantages over other public contributors; for example, in studies of new interventions or procedures, and where it is hard to identify or reach patient or community groups that include or speak for the study population. Participant involvement is possible at all stages of a trial; however, because there are no participants to involve during the design stage of a trial, prior to enrolment, participant involvement should complement and not replace involvement of PPI stakeholders. A range of models, including those with managerial, oversight or responsive roles are appropriate for involving participants; however, involvement in data safety and monitoring committees may not be appropriate where there is a potential risk of unblinding. Involvement of participants can improve the trial experience for other participants; optimising study procedures, improving communications; however, there are some specific, notably, managing participant confidentiality and practicalities relating to payments. CONCLUSIONS: Participant involvement in clinical trials is feasible and complements other forms of PPI in clinical trials. Involving active participants offers significant advantages, particularly in circumstances where trials are assessing new, or otherwise unavailable, therapies or processes. We recommend that current guidance on PPI should be updated to routinely consider including participants as valid stakeholders in PPI and potentially useful approach to PPI.