Positron Emission Tomography (PET) permits the non-invasive quantification of myocardial blood flow and flow reserve. This capability requires an understanding of the determinants of resting and hyperemic blood flow in order to interpret correctly quantitative findings with PET. Equally important is the knowledge of how such measurements are obtained and of their advantages and limitations. The aim of the current overview was therefore a) to review the concept of the myocardial flow reserve and b) to discuss recent animal experimental and clinical studies which have employed quantitative PET for measuring myocardial blood flow and flow reserve.