Kr-81m and Xe-133 are inert (noble) gases. Kr-81m has an ultra-short half-life of 13 sec. It is the generator-produced, metastable daughter of Rb-81, which decays by electron capture to stable Kr-81 via Kr-81m. Kr-81m is the optimum tracer for VQ lung scintigraphy. The steady state count rate recorded during continuous inhalation portrays the distribution of ventilation. In contrast, continuous inhalation of Xe-133, which has a half-life of 5.2 days, gives an image of regional lung volume. The tissue washout rate of Xe-133 administered intra-arterially measures tissue perfusion without the complication of re-circulation. Other agents for ventilation imaging are Tc-99m-DTPA aerosol and Technegas. The hallmark of pulmonary embolism in VQ scintigraphy is a perfusion defect that is not matched by a corresponding defect in ventilation. Chronic pulmonary embolism is unresolved acute embolism and has the same scintigraphic appearances. There are several other causes of mismatched perfusion defects, including interstitial lung disease, lung cancer and, in paediatrics, sequestered lobe. Parenchymal lung disease, such as COPD, gives matched defects of perfusion and ventilation.