This chapter, written for undergraduate medical students and post-graduate doctors in training, presents information on how to assess the patient's mental capacity — a case which they are likely to encounter in the future in a general practice setting. It presents the case of a 76-year-old retired aircraft engineer who comes to a routine appointment. The husband is visibly upset and asks for a letter to his solicitor confirming that he has the capacity to manage his financial affairs. The solicitor told him that the first step was to visit the general practitioner (GP) so that he or she can confirm his mental capacity. The chapter provides details of the patients' medical history and the key findings of clinical examination, together with initial investigation results data for evaluation, and helps them develop their skills of clinical reasoning. Key questions then prompt the student to evaluate the patient, and reach a decision regarding their condition and the possible treatment plan.