Abstract: The invention relates to an illumination system (10), a backlighting system and a display device. The illumination system comprises a light source (20) configured to emit light via a light guide (30) to a light exit window (40) of the illumination system. The light guide has a front wall (32) arranged opposite a rear wall (34) so as to guide light in a direction substantially parallel to the front wall. A distance (D) between the light guide (30) and the light exit window (40) reduces towards an edge (42) of the light exit window (40). The light guide (30) further comprises a light entrance window (36) for receiving the light from the light source which is arranged away from the edge of the light exit window. The illumination system according to the invention has, inter alia, the effect that its thickness may be reduced at the edge of the light exit window.