This chapter, written for undergraduate medical students and post-graduate doctors in training, presents information on how to manage gynaecomastia — a case which they are likely to encounter in the future in a general practice setting. It 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. The case is about a 12-year-old boy who has a lump on his chest. True gynaecomastia is a persistent, usually bilateral, enlargement of breast tissue in males. In this case, a quick trawl through his records shows that he has had his measles, mumps and rubella (MMR) immunizations and has not had mumps. A simple examination of his genitalia shows normal testes and penis, excluding rarities such as Klinefelter's syndrome.