Coronary microvascular dysfunction (CMD) involves impaired function of coronary arterioles (< 500 μm) that cannot be visualized on standard coronary angiography. The pathophysiology includes endothelial dysfunction with reduced nitric oxide bioavailability, smooth muscle dysfunction with impaired vasodilation, microvascular remodeling with increased wall-to-lumen ratio, and extravascular compressive forces. CMD is the underlying mechanism in many patients with angina and non-obstructive coronary arteries (ANOCA/INOCA), affecting up to 50% of patients referred for angiography who have no significant epicardial stenosis. The coronary flow reserve (CFR < 2.0) and index of microcirculatory resistance (IMR > 25) are invasive measures of microvascular function. CMD is associated with adverse cardiovascular outcomes including MI, HF, and CV death despite normal angiography.