While Coronary angiography remains the standard for the diagnosis and treatment of coronary artery disease within the cardiac catheterization lab, many lesion subsets require further investigation to determine their overall significance in causing symptoms. Direct measurement coronary flow and subsequently the ability to measure coronary pressure changes at maximal hyperemia have been developed to provide physiologic data to the catheterization operator which reveal the significance of coronary artery lesions in question and guide treatment. These measures, coronary flow reserve (CFR) and fractional flow reserve (FFR) have been validated against non-invasive tests for myocardial ischemia. Furthermore, FFR has also been studied in its ability to guide percutaneous coronary interventions in single vessel disease, multi-vessel disease, ostial side-branch lesions, and left main lesions.