Abstract. Internal Transport Barriers (ITB's) in tokamak plasmas are a promising way to achieve steady-state plasmas with good confinement properties in a fusion reactor. Despite a large activity in this field, the formation and self-sustainment of barriers are still not well understood. The physics of ITB's in JET has been investigated with various techniques, namely micro-stability analysis, profile modelling and turbulence simulations. The calculation of linear growth rates show that the magnetic shear plays an important role in the formation of the ITB. The Shafranov shift, ratio of the ion to electron temperature, and impurity content further improve the stability. However the ExB velocity shear is important for the sustainment and the motion of the barrier. This picture is consistent with profile modelling and global fluid simulations of electrostatic drift waves. Turbulence simulations also show that the formation mechanisms are different for electron and ion barriers. Ion barriers are mainly due to the formation of a gap in the density of low wavenumber resonant surfaces whereas electron barriers are rather due to a reversal of the precession drift of trapped electrons.