Regional pulmonary perfusion and ventilation can be assessed by imaging, with positron emission tomography (PET), the pulmonary kinetics of [13N] nitrogen (13N2). Because of its low solubility in blood and tissues, 13N2 infused intravenously in saline solution evolves into the alveolar airspace at first pass, where it accumulates in proportion to regional perfusion during a short apnea. In contrast, infused 13N2 is not retained in non-aerated regions, which do not exchange gas. Robust estimates of regional perfusion and shunt are obtained by modeling the pulmonary kinetics of 13N2 infused as a bolus during a short apnea. Regional ventilation is measured by modeling the washout of 13N2 after breathing is resumed. Regional gas content and dead space ventilation can be measured with inhalation of 13N2. Application of this novel functional imaging technique can further the understanding of the pathophysiology of a variety of pulmonary processes. This review briefly describes the methodological aspects of PET imaging of regional perfusion and ventilation and then focuses on insights in the pathophysiology of acute lung injury and asthma that have been gained by imaging the pulmonary kinetics of 13N2 with PET.