Presence, extent, and evolution of atherosclerotic coronary narrowings, as well as the anatomic possibilities for revascularization, can be reliably defined at (and only at) selective coronary angiography. The latter remains, therefore, the pivotal diagnostic tool for patients with suspected coronary artery disease. However, in spite of the increasing availability of on-line quantitative coronary angiography, it still holds that the functional (physiologic) consequences of an epicardial coronary narrowing cannot be completely derived from geometric (anatomic) information. Clinical decision making can be particularly difficult in lesions of intermediate severity (40-70% diameter reduction), in postinterventional segments, and in some particular anatomic settings, namely, ostial stenoses, bifurcation lesions, and diffuse atherosclerotic disease. This has led to an explosive growth of new methods for assessing the physiological significance of coronary narrowings documented at angiography. Among them, Doppler blood flow velocitometry and transstenotic pressure gradient measurements have emerged as the only techniques easily applicable in most catheterization laboratories. Here, we briefly review the clinical interest of measuring transstenotic pressure gradients.