Radioactive gases are useful for evaluating pulmonary ventilation because they allow assessment of regional lung function in a physiologic, noninvasive manner. The most widely used radioactive gases are Xe-133, Xe-127, and Kr-81m. These gases provide visual and/or quantitative assessments of regional ventilation or ventilation-perfusion matching in obstructive airways disease, and aid the scintigraphic diagnosis of pulmonary embolism. The precision of ventilation scintigraphy is limited by detector resolution, signal distortion at depth, poor counting statistics, and respiratory motion.