Thiso chapter presents aircraft-based methods of measuring the flux densities of sensible and latent heat, carbon dioxide, ozone, nitrous oxide, methane, and other trace gases. The main techniques and sensors that are used to measure flux densities with an aircraft are briefly described. Factors that affect the accuracy of those flux density measurements are discussed, including analysis techniques, run lengths, sampling heights, surface and environmental conditions, and data quality assessment. The use of aircraft-based flux density measurements to evaluate the representativeness of tower-based flux measurements is examined. The versatility of aircraft to act as sensor platforms under a wide range of conditions is demonstrated using several interesting examples. Future potential research directions are mentioned.