Radionuclide myocardial perfusion imaging (MPI) is among the most commonly performed diagnostic tests in cardiology. Although the diagnostic and prognostic applications of radionuclide MPI are supported by a wealth of observational and clinical trial data, its performance is limited by two fundamental drawbacks. First, conventional MPI by SPECT and PET measures relative perfusion, that is, the assessment of regional myocardial perfusion relative to the region with the highest perfusion tracer uptake. This means that a global reduction in myocardial perfusion (‘‘balanced’’reduction of flow) may remain undetected and that, in general, the extent of coronary artery disease (CAD) is underestimated, as has been demonstrated with both 201Tl-and 99mTc-labeled perfusion tracers. 1–3 For example, Lima et al. found that in patients with severe 3-vessel CAD, 99mTc-sestamibi SPECT MPI showed perfusion defects in multivessel and typical 3-vessel-disease patterns in only 46% and 10% of patients, respectively. 2 Similarly, it has been reported that only 56% of patients with left main CAD are identified as being at high risk by having more than 10% of the myocardium abnormal on stress SPECT MPI. 4 Second, the 99mTc flow tracers available for SPECT MPI are inherently limited by a relatively low first-pass extraction fraction at high flow rates, thus limiting the precision and accuracy of these tracers for estimation of regional myocardial blood flow (MBF) during stress. 5 Clinical studies have shown that even small differences in extraction fraction can result in a clinical difference in the detection and quantification of myocardial ischemia by SPECT. 6, 7 These drawbacks of SPECT are addressed by PET, with its ability to quantify global and regional MBF (in mL/minute/g of tissue), assess regional perfusion abnormalities with relative MPI, and assess contractile function abnormalities and chamber dimensions with gated imaging. The purpose of this document is, first, to consolidate and update technical considerations for clinical quantification of MBF and myocardial flow reserve (MFR) from earlier documents 8 and, second, to summarize and update the scientific basis for their clinical application. 9, 10