Abstract: An air diffuser embodying a housing, defining an air chamber, adapted to be mounted in a suitable opening in a wall or ceiling of a room, and having an air inlet in one side thereof adapted to be connected to an air duct, or the like, and an air outlet on the opposite side thereof, with a substantially flat air deflector unit extending across the air outlet and defining the outer face of the air diffuser remote from the air inlet, and deflector unit having a plurality of adjustable modules having openings constituted and arranged therein in such a manner as to cause air discharged from the outlet to flow laterally away from the diffuser along the wall or ceiling in which the diffuser is mounted.