ZnO layers have been grown by atomic layer epitaxy using a gas flow version of the technique. ZnO films have been obtained from either a double exchange chemical reaction, a single exchange reaction, or from elemental components, i.e., from zinc and oxygen. We have also studied ZnO layers prepared by the oxidisation of ZnS layers. Silicon ([001] and [111]), GaAs, sapphire, sapphire/GaN or soda lime glass substrates have been used. We demonstrate that ZnO films are suitable as buffer layers for GaN epitaxy.