The aim of this review is to highlight the role of invasive functional evaluation in patients in whom coronary artery bypass graft (CABG) is indicated, and to examine the clinical evidence available in favour of fractional flow reserve (FFR) adoption in these patients, outline appropriate use, as well as point out potential pitfalls. FFR after CABG will also be reviewed, highlighting its correct interpretation and adoption when applied to both native coronary arteries and bypass grafts. Practice European guidelines support the use of FFR to complement coronary angiography with the highest degree of recommendation (Class IA) for the assessment of coronary stenosis before undertaking myocardial revascularization when previous non-invasive functional evaluation is unavailable or not conclusive. As a result, FFR has been adopted in routine clinical practice to guide clinicians decision as to whether or not perform a revascularization. Of note, due to the increasing confidence of the interventional cardiologists, FFR guidance is also being implemented to indicate or guide CABG. This is in anticipation of supportive clear-cut evidence, since recommendations for FFR adoption were based on randomized clinical trials investigating percutaneous coronary intervention (PCI) strategies in which patients with typical indications for CABG were excluded (e.g. left main disease, valvular disease, and coronary anatomy unsuitable for PCI). Based on the critical appraisal of the literature, FFR can play an important role in risk stratification and determining management strategy of patients either before or after CABG.