Unstable angina (acute coronary insufficiency; pre-infarction angina; worsening angina; intermediate syndrome), characterized by a progressive increase in anginal symptoms, new episodes of rest or nocturnal angina or the appearance of prolonged anginal duration. Unstable angina (unstableangina) is a clinical manifestation between exertional stable angina and acute myocardial infarction and sudden death. It mainly includes primary angina, worsening exertional angina, resting angina with electrocardiographic ischemic changes and early angina after myocardial infarction. Because of its unique pathophysiological mechanism and clinical prognosis, patients may develop acute myocardial infarction if they are not treated appropriately and timely. Unstable angina is secondary to acute exacerbation of coronary obstruction, the latter being caused by rupture of the fibrous plaque on the surface of the atheroma and consequent platelet adhesion. More than 1/3 of patients with unstable angina have angiographic confirmation of thrombus leading to partial occlusion in the vessels of the ischemic zone, and the reported incidence may be low because it may be difficult to identify the thrombus at the time of angiography. Compared with stable angina, unstable angina is more painful and longer lasting, can be induced by lower activity levels, can occur spontaneously at rest (prone angina), is progressive in nature (worsening), and these changes can occur in any combination. Myocardial infarction may occur within 3 months after the attack in about 30% of patients with unstable angina. Sudden death is rare, and significant changes in the ECG during chest pain are an important marker for the development of myocardial infarction and sudden death.