The evaluation of sudden visual loss should begin with the differentiation between monocular loss and binocular loss. The importance of this is reflected in the differences between the main causes of monocular and binocular losses. In cases of transient monocular visual loss, an ocular cause has to be kept in mind so as to avoid unnecessary and costly cerebrovascular investigations. In cases of persistent monocular visual loss, a compressive lesion of the optic nerve or chiasma may simulate optic neuritis. In the evaluation of diplopia, the main differential diagnoses are nerve lesions and myasthenia. The main causes of nerve lesions responsible for diplopia and their workup are summarised. The usefulness of eye signs in the diagnosis of myasthenia is highlighted. The possibility of compressive lesions co-existing with or masquerading as myasthenia is emphasised.