Several observational data indicate that type 2, narrow line Seyfert nuclei suffer significant obscuration along our line of sight. The unified model (Antonucci 1993) ascribes this obscuration to a gaseous parsec-scale circumnuclear torus. According to this model, broad line Seyfert 1 (Sy1) and narrow line Seyfert 2 (Sy2) galaxies would be identical physical objects, while the orientation of the line of sight with respect to the torus is responsible for the obscuration of the Broad Line Region (BLR) and of the nuclear engine (X-UV source) in Sy2s.
In this review I show how X-ray observations, especially at high energies (> 2 keV), provide a wealth of information which constrain the properties of the absorbing medium in Seyfert 2s, i.e. the putative circumnuclear gaseous torus. I also discuss the implications of these X-ray studies on related issues, such as the fuelling of active nuclei and the X-ray background. Due to the limited space, I do not discuss warm absorbers, although evidence for this component is found in some obscured Seyferts (eg. Komossa & Fink 1997), neither I discuss the soft excess that characterizes most Sy2s (Maiolino et al. 1998, Wilson & Elvis 1997).