Atomic layer etching (ALE) has been known for more than twenty years, but has recently come to prominence in semiconductor device manufacturing, where it may address some difficult etch problems. The basic principles of an ideal ALE process are described, together with some practical limitations of putting it into practice. The applications that are likely to benefit from ALE are discussed, and results given for etching silicon, silicon dioxide, and a ‘two dimensional semiconductor’ molybdenum disulphide.