Coronary artery disease (CAD) is a leading cause of morbidity and mortality. While cardiologists have been focused on discrete, visible stenoses of coronary arteries, there is increasing awareness of the importance of microcirculation and vasospastic disorders in causing angina. The microvascular bed is composed of vessels smaller than 400 microns in diameter. Their network is significantly larger than that of the epicardial vessels and serves essential functions, including regulating myocardial blood flow and cellular metabolism. Angina pectoris, a most frequent symptom of CAD or myocardial ischemia, was assumed to be caused by significant stenosis of the epicardial coronary artery. However, it was found that in over 50% of cases, there was no obstructive CAD, which is described as angina with no obstructive coronary arteries (ANOCA) or ischemia with no obstructive coronary arteries (INOCA), according to the clinical setting.