Although various angiographic or flow criteria are currently used to evaluate the severity of side branch lesions, none of these has been validated yet. Moreover, angiographic evaluation alone is sometimes inaccurate and does not reflect the functional severity of stenosis, especially in ostial lesions. Fractional flow reserve (FFR) is an easily obtainable lesion-specific parameter for the physiologic evaluation of epicardial coronary artery stenosis, which takes into account the interaction between the anatomic stenosis and the area of perfusion supplied by a specific coronary artery. Recently, a series of investigations was performed to compare the functional severity and angiographic severity in side branch lesions and to evaluate the functional outcomes of jailed side branch lesions during follow-up. From these studies, it was found that 1) FFR-guided provisional side branch intervention strategy is feasible and effective, 2) angiographic evaluation overestimates the functional severity of jailed side branch lesions in every step of the provisional strategy for bifurcation lesions and 3) functional status of jailed side branch lesions after drug-eluting stent implantation does not change significantly during follow-up.