An illumination system for walls, ceilings or floors comprising: - a support structure (10, 100) extending in a longitudinal direction and forming an internal space accessible from an opening of said support structure (10, 100), wherein the support structure comprises two facing side walls that define two internal side surfaces and a rear wall connecting the two facing side walls and closing the internal space at the rear portion thereof, - at least one functional unit extending in the longitudinal direction which can be housed in the internal space of the support structure (10, 100), wherein the functional unit comprises at least one electrical contact; - at least one illumination device (6, 7, 8) provided with illumination means (106, 206, 306) and comprising electrical contacts that can be coupled to the at least one electrical contact of the functional unit to keep the illumination device (6, 7, 8) connected to the functional unit, wherein the at least one functional unit is integrated in at least one of the two side walls of the support structure (10, 100) and it is intended to face the electrical contacts of the illumination device (6, 7, 8), characterised in that the at least one functional unit comprises a plate of conductive material, the plate of conductive material comprising an outer surface wherein the at least one electrical contact is arranged in the form of a darkened conductive coating, and at least one electrical connector arranged in a profile of insulating material, on the opposite face of the outer surface of the functional unit, whereby the profile of insulating material is arranged in a side groove present in at least one of the two internal side surfaces of the two side walls of the support structure (10, 100), wherein the functional unit is arranged inside said profile of insulating material, such that the outer surface of the functional unit defines the internal space in width. - a support structure (10, 100) extending in a longitudinal direction and forming an internal space accessible from an opening of said support structure (10, 100), wherein the support structure comprises two facing side walls that define two internal side surfaces and a rear wall connecting the two facing side walls and closing the internal space at the rear portion thereof, - at least one functional unit extending in the longitudinal direction which can be housed in the internal space of the support structure (10, 100), wherein the functional unit comprises at least one electrical contact; - at least one illumination device (6, 7, 8) provided with illumination means (106, 206, 306) and comprising electrical contacts that can be coupled to the at least one electrical contact of the functional unit to keep the illumination device (6, 7, 8) connected to the functional unit, wherein the at least one functional unit is integrated in at least one of the two side walls of the support structure (10, 100) and it is intended to face the electrical contacts of the illumination device (6, 7, 8), characterised in that the at least one functional unit comprises a plate of conductive material, the plate of conductive material comprising an outer surface wherein the at least one electrical contact is arranged in the form of a darkened conductive coating, and at least one electrical connector arranged in a profile of insulating material, on the opposite face of the outer surface of the functional unit, whereby the profile of insulating material is arranged in a side groove present in at least one of the two internal side surfaces of the two side walls of the support structure (10, 100), wherein the functional unit is arranged inside said profile of insulating material, such that the outer surface of the functional unit defines the internal space in width.