As feature sizes of semiconductor devices diminish to the atomic scale, their fabrication also requires an atomic-scale accuracy. Atomic layer etching (ALE) is a dry etching process having an atomic-scale resolution. In ALE, a thin layer of a material surface is removed sequentially by a cycle of two reaction steps, i.e., the formation step of surface modification and the removal step of a surface layer affected by the surface modification. In plasma-assisted ALE (PA-ALE), plasmas are used to generate reactive species that modify the surface and/or low-energy ions that assist the removal of the modified surface layer. In this review article, the interactions of halogen atoms and hyper-thermal halogen molecules with a silicon (Si) surface are discussed, which correspond to typical surface reactions in the formation step of PA-ALE of Si. Similarly, the interactions of halogenated Si surfaces with low-energy inert-gas incident ions are also examined as typical surface reactions of the removal step of PA-ALE of Si.