Fractional flow reserve (FFR) is an invasive physiologic index measured in the cardiac catheterization laboratory to assess the functional significance of a coronary stenosis. It is now accepted as the reference standard to indicate whether a stenosis is likely to be responsible for ischemia. It is generally accepted that a stenosis with an ischemic value of FFR is responsible for symptom and a worse outcome and should be revascularized, whereas lesions with a non-ischemic FFR have a more favorable prognosis and can be treated medically. Furthermore, FFR-guided revascularization strategy has been definitely proven to be better than angiography-guided strategy in pivotal landmark studies such as DEFER, FAME, and FAME2. Instantaneous wave-free ratio (iFR) is another physiological index which can be obtained at rest without hyperemic stimulation. iFR is conceptually different from FFR, leading to lively scientific debate about this index. In this review article, the concept, clinical value of FFR and iFR for clinical practice are reviewed.