Cardiac tamponade is a medical emergency that can be readily reversed with timely recognition and appropriate intervention. The clinical diagnosis of cardiac tamponade requires synthesis of a constellation of otherwise nonspecific features based on an understanding of the underlying pathophysiological characteristics. Although echocardiographic examination is a central component of diagnosis, alone it is insufficient to establish the physiological diagnosis of hemodynamically significant cardiac tamponade. The hemodynamic diagnosis of cardiac tamponade requires clinical evidence of low cardiac output and stroke volume in the setting of elevated cardiac filling pressures, with evidence of increased sympathetic tone (eg, tachycardia, peripheral vasoconstriction), and exclusion of other causes of shock as the primary problem (particularly cardiogenic shock). The hemodynamic features of tamponade are revealed by considering the effects of pericardial constraint. Pulsus paradoxus and loss of the normal "y" descent of a jugular venous pressure waveform may be appreciated on clinical examination. When a pulmonary artery catheter is placed, equalization of diastolic pressures across all chambers is observed. Echocardiographic examination confirms the size, location, and other characteristics of the causal pericardial collection. Several echocardiographic features support the hemodynamic diagnosis of tamponade, including early diastolic collapse of the right ventricle, late diastolic collapse of the right atrium, respiratory variation in mitral valve inflow (akin to pulsus paradoxus), and decreased early filling (E wave) of mitral valve inflow (related to loss of the y descent). Echocardiographic examination then supports decisions about the early treatment and drainage of the tamponading effusion.