Atomic layer deposition (ALD) is a thin film deposition technique that is based on the sequential use of a gas phase chemical process. ALD is considered a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals, typically called precursors. These precursors react with the surface of a material one at a time in a sequential, self-limiting, manner. Through the repeated exposure to separate precursors, a thin film is slowly deposited. ALD is a key process in the fabrication of semiconductor devices, and part of the set of tools available for the synthesis of nanomaterials.