The diagnosis of dementia does not mean that a person is by definition incompetent to consent. Although the legislation has been modified to allow for research on incompetent persons, still, it becomes increasingly important to be able to judge the capacity to consent on an individual basis. Classically, what is considered necessary at least for competent judgement is: (1) ability to receive and understand information; (2) ability to process information; (3) ability to appreciate the situation and its consequences; (4) ability to weigh benefits, risk and alternatives; (5) ability to make and communicate a decision. The best validated instrument currently available for evaluation of competency is the MacArthur Competency Assessment, which can be applied both for research aims and for clinical practice decision making. If an individual is unable to provide informed consent, proxy (e.g. family) or double consent are alternatives. The patient’s behaviour should be closely monitored and patients who demonstrate objection or signs of refusal should not be included and excluded once the study has started. Ultimately, application of the best competency assessment instrument, which is asking the right questions to check for competency on specific issues, should be combined with knowledge of the patient’s hopes, beliefs and personal history. Combining these elements will give both researchers and medical doctors the best chances for an ethically justified answer on how to offer dementia patients a realistic opportunity to benefit from participating in clinical research, but still protect their autonomy, and sufficiently recognize their vulnerability to prevent harm.