Angle-closure disease is less common than open-angle glaucoma worldwide, but its incidence in certain populations is known to be much higher than previously documented. Also, its impact on quality of life of the patient is more critical due to a greater likelihood of blindness, and higher morbidity rates than in patients with open-angle glaucoma. An accurate and timely diagnosis is therefore essential, in order to prevent progression to irreversible visual field loss. This chapter covers the classification, mechanisms, strong index of suspicion, and clinical examination and pitfalls. The diagnosis of acute PAC is mainly clinical. Diagnosis requires a careful history to assess risk factors, slit lamp examination including IOP and gonioscopy.