The TExas and FLorida UNderflights (TEFLUN) Experiment is a mission to obtain validation measurements for the Tropical Rain Measuring Mission (TRMM). TRMM is a NASA and National Space Development Agency of Japan (NASDA) coordinated mission that launched the TRMM satellite on 28 November 1997 with a unique complement of sensors to remotely observe rainfall throughout the global tropics. TEFLUN is the first in a series of experiments using a combination of airborne and surface-based measurements to complement the satellite data. Among these, are important measurements aboard the NASA high-altitude aircraft, similar to those on the TRMM satellite. They are used for direct intercomparisons with TRMM overflights where possible, but more frequently to simulate TRMM data by flying over precipitation systems within the experimental domain. These, along with in situ aircraft data, surface-based measurements and computer models, will make unique contributions to our understanding of the tropical precipitation spectrum.