Abstract: We often forget in our daily life that air does not have the same optical properties as vacuum. At least in New Mexico and Arizona, we are made aware that it has an index of refraction, and that it is not the ideal homogeneous optical material. However, in daily experiments, we do not think too often of air as being a nonlinear medium, having a complex intensity dependent index of refraction, nonlinear absorption, induced birefringence, and becoming a partially conductive medium. These properties lead to light filamentation, a situation where the nonlinear properties of air dominate the propagation properties. It has produced — and still is producing — a flurry of papers and dreams of wild applications. Aside from the practical or unpractical applications, it is a unique example of light-matter-light interaction, which makes us rethink basic concepts of electromagnetism, even down to the nature of an index of refraction. He investigating two types of filaments, produced either by femtosecond pulses in the near infrared (800 nm) and by nanoscond pulses in the ultraviolet (266 nm). The two are interesting in comparison because of their very different wavelength and temporal regime. An infrared filament is an ideal object of study for investigating strong field light-matter interaction, in which light and matter have a mutual recordable effect on each other. For a few hundred femtosecond-long infrared filament in air, the interaction of light is with bond electrons in atoms or molecules, with free electrons created by tunnel ionized, and with partially orientated molecules. Since the modification of light happens in a time scale much faster than a plasma period, a careful microscopic (in the fs scale) study of the parameters involved in filament formation is needed. In this talk, Diels shows how pre-filamentation propagation can cause a) new spectral development and b) polarization-dependent filamentation. We used an aerodynamic window to prepare the focus in vacuum before launching the filament in air. A model to study the index of refraction of tunneled electrons in the femtosecond time scale of a laser pulse was presented.