Dementia, which affects about 5% of those aged over 65 years, is the progressive decline in cognitive function due to damage or disease in the body beyond that expected from normal aging. It causes the loss of mental abilities such as thinking, remembering and reasoning and memory impairment, which is important when considering informed consent in radiotherapy. Radiotherapy treatment often involves many visits for a patient and each time it is the legal responsibility of staff to ensure that patient is aware of what is happening and why. Informed consent is challenging in patients with dementia because of the memory impairments and the nature of the information. There is enormous variation in how this illness affects people, depending on the type of dementia and the individuals themselves. Full information about the treatment itself, the benefits and risks of the treatment and the availability of any other reasonable alternatives to the proposed treatment or procedure must be made available. Dementia patients respond well to set approaches and these are detailed. It is important for staff to be familiar with the most effective approaches and consider these in obtaining informed consent.