AbstractConventional atomic layer deposition (ALD) is a thermo-chemical process that is now used extensively in the manufacture of ultrathin films. In addition to substrate heating, various forms of other “assisted” ALD processes are actively being developed, where supplementary energy is supplied for example, from a plasma discharge or from light. This paper presents a critical review of the exploitation of light in ALD to stimulate photochemical processes. The range of light sources that are exploitable for photochemical ALD processes is considered and the chemical mechanisms that are stimulated in the ultraviolet spectrum are interpreted. The use of light as an excitation source lends itself to area selective deposition using lithographic methods or focused beams. The exploitation of photochemical processes for the deposition of patterned ALD films is reviewed in the context of the current alternatives. Finally, the potential for photo-etching is introduced. Atomic layer etching is a complimentary process to ALD and the application of photochemistry in layer-by-layer subtraction processes is considered.