In every case knee injury, it is imperative not to miss a possible lesion of the ligaments and, when such a lesion is present, to evaluate its severity. The practitioner who examines the injured knee in an emergency therefore plays a crucial role in taking a definite attitude when faced with lesions. Before any treatment is envisaged the lesion must be precisely diagnosed. This requires a very strict examination based on the patient's history and on clinical tests that are often simple. At the slightest suspicion of severe lesion of the ligaments, a specialist must be called in to complete the investigation, if necessary by testing under general anaesthesia and/or by performing an arthroscopy.