Abstract: The fate of an ultrashort laser pulse propagating in air depends crucially upon its peak power. Below a critical value, Pcr, group velocity dispersion and beam diffraction combine to rapidly reduce the pulse intensity. On the other hand, if P is less than Pcr, a completely different behaviour is observed. In this case, instead of decreasing, the pulse intensity increases with distance up to the point where it becomes sufficiently high (~1013 W/cm²) to ionize air. The pulse then retains this high intensity for very long distances that can reach kilometers. This regime is called filamentation. In this lecture the basic notions at the heart of filamentation will be introduced. Techniques to characterize air filaments will be described. This includes measurements of the beam size, pulse intensity, pulse duration, density and length of the plasma column created in the wake of the pulse, and the plasma density evolution. These results are well reproduced by numerical simulations. Recent experiments will be described which allow to manipulate and to exchange energy between filaments. A second part will be devoted to applications of filaments. They include the triggering and guiding of low resistance, high current electric discharges, the creation of short bursts of terahertz radiation, the illumination of distant objects, the use of filaments as virtual RF antennas. Presented Thursday, October 11, 2012.