Different laser devices working as "atom diodes" for ultra-cold atoms have been proposed recently. They transmit ground state level atoms coming from one side, say from the left, but reflect them when they come from the other side. We present and compare some of these models. In all of them spontaneous decay is included as an irreversible element which avoids backwards motion after the atom has crossed the diode. We also review the proposal of a cooling method based on an "atom diode".