Fractional flow reserve (FFR) has become an increasingly important index for decision making with respect to revascularization of coronary artery stenosis. It is the gold standard to indicate whether a particular stenosis is responsible for inducible ischemia and it is generally accepted that a stenosis with an ischemic value of FFR is responsible for angina pectoris and a worse outcome, and should be revascularized, whereas lesions with a non-ischemic FFR have a more favorable prognosis and can better be treated medically. In this review paper, the background, concept and clinical application of FFR are discussed from a practical point of view. On top of that, some in-depth considerations are given with respect to further possibilities of FFR for examining the coronary circulation, including separate assessment of coronary, myocardial, and collateral blood flows. Finally, a word of caution is given with respect to using resting pressure indexes, which seem attractive because they avoid the need for hyperemia, but negatively affect the accuracy of the measurements. This review can be read as an overview of the state-of-the-art of FFR and as a guide to further reading.