To make a valid will, a person should be able to understand the nature and consequences of doing so, the extent of their estate and the claims others may have on it. No disorder of mind should be present that would affect their testamentary decisions, and clinicians are therefore often asked to give an opinion on whether a person has testamentary capacity. This article discusses the legal issues involved, with reference to UK case law (in particular, the legal test of Banks v Goodfellow (1870)), and outlines the requirements of testamentary capacity assessment (including retrospective assessments), the clinician's responsibilities when requested by a solicitor to make an assessment of capacity (‘the golden rule’) and what they might expect if appearing in court to give expert witness regarding testamentary capacity. Fictitious case studies are presented illustrating certain points in testamentary capacity assessment.