Fractional flow reserve assessment was accepted as a crucial strategy in stable patients undergoing coronary angiography without prior noninvasive evaluation in the presence of borderline lesions and in patients with multivessel coronary artery disease. Instantaneous wave-free ratio, measured during a specific diastolic interval, emerged as a nonhyperemic pressure ratio. Due to its advantages such as not requiring a vasodilating agent, rapidity of procedure, pullback phenomena for the assessment of individual stenosis in tandem lesions or diffusely infiltrated vessels, and virtual percutaneous coronary intervention which allows the assessment and justification 'of' optimal coronary revascularization, instantaneous wave-free ratio became a valuable option in the field of coronary physiology. This review aims to address coronary physiological concept with fractional flow reserve and emergence of instantaneous wave-free ratio through cornerstone studies as well as the use of instantaneous wave-free ratio in different clinical scenarios.