Functional evaluation of bifurcation lesions is more difficult than usual lesions due to their complex anatomy. Angiographic and intravascular ultrasound criteria for main branch intervention cannot be directly applied to side branch lesions due to the difference in underlying lesion characteristics, geometric changes during intervention, and the size of myocardial territory. Fractional flow reserve is a physiologic parameter which reflects both the degree of stenosis and the area of perfusion supplied by a specific coronary artery. The present review will focus on using fractional flow reserve in bifurcation lesions.