The index of microcirculatory resistance (IMR) can be measured readily and simultaneously with fractional flow reserve (FFR), and is a comprehensive integrated assessment for the epicardial and microvascular compartments of the coronary circulation. In contrast to other invasive and non-invasive tests, this method is known to be stable and reproducible under various hemodynamics and even in the presence of coronary artery stenosis. This review summarized the theoretical basis, operation skills, and the clinical practice of IMR.