2.0 BACKGROUND Angiographic severity of coronary artery stenosis has historically been the primary guide to mechanical revascularization and/or medical management of coronary artery disease. Studies have shown that in patients with stable coronary artery disease (CAD) percutaneous coronary intervention (PCI) based on angiographic stenosis severity does not reduce coronary events more than initial medical treatment. In contrast, randomized trials of PCI report better outcomes when guided by fractional flow reserve (FFR) than when guided by angiographic severity or when compared to initial medical treatment.