The end-product of this research (a practice guideline for how to assess decision-making capacity among brain cancer survivors) will be for healthcare workers to use. However, it is essential that people with cancer and informal caregivers are involved at each stage of making this practice guideline so that it is sensitive, relevant, and acceptable to people with brain cancer and their families. The PPI collaborators will form a PPI advisory panel for this project. The panel will meet for a 1-hour meeting every 4-5 months, to advise on research questions, how to collect data, how to facilitate cancer survivor/caregiver participation, how to interpret some data, and how to disseminate results at each stage. The first stage of the research consists of doing a scoping review – this is where the researchers look at all the published studies/existing guidelines on this topic and identify gaps in our knowledge. PPI collaborators have been asked to help develop the questions for this review and guide what studies we include/exclude. Next, the research team will interview key stakeholders (cancer survivors, caregivers, healthcare professionals, solicitors) about their knowledge and attitudes towards the new decision-making capacity law. PPI collaborators will assist with the vital task of formulating the interview questions (content/topic of the questions/phrasing) for participants and advising on how to facilitate cancer survivors and caregivers in participating (where to interview them, how long the interview should be, etc). PPI collaborators may also assist with data analysis for the survivor interviews (interpreting what the survivors said). Finally, the research team will recruit cancer survivors, caregivers, healthcare professionals, and solicitors to participate in co-design of the practice guideline for decision-making capacity assessment. Co-design is a type of research where key stakeholders come together to find a solution to a shared problem. This process will take place over 12 months with key stakeholders coming together to decide what information identified in the previous steps should be included in the guideline/how to present the information. Co-design is separate from PPI and caregivers and survivors involved in the co-design are participants and will be separate from the PPI collaborators on the PPI advisory panel. PPI collaborators will advise on how to structure the co-design meetings, how often to meet, how to facilitate cancer survivor and caregiver participation (so their views are considered on an equal basis with healthcare professionals/solicitors). PPI Collaborators will be invited to co-present findings at a national conference.