Pulsus paradoxus is one of the cardinal signs of cardiac tamponade and must be looked for at bedside examination of any patient who presents a clinical picture of low cardiac output. This paper reviews the definition of pulsus paradoxus and the way to measure it noninvasively. We then discuss the physiological fluctuation of systemic blood pressure during respiration and the various mechanisms which lead to an exaggeration of this phenomenon during tamponade. The sensitivity and specificity of this sign are also discussed. Finally, the role of echocardiography in the diagnosis of cardiac tamponade is shortly presented, as this method appears to be very accurate and sensitive in evaluating the hemodynamic embarrassment associated with pericardial effusion.