Abstract This article discusses key elementary surface-reaction processes in state-of-the-art plasma etching and deposition relevant to nanoelectronic device fabrication and presents a concise guide to the forefront of research on plasma-enhanced atomic layer etching (PE-ALE) and plasma-enhanced atomic layer deposition (PE-ALD). As the critical dimensions of semiconductor devices approach the atomic scale, atomic-level precision is required in plasma processing. The development of advanced plasma processes with such accuracy necessitates an in-depth understanding of the surface reaction mechanisms. With this in mind, we first review the basics of reactive ion etching (RIE) and high-aspect-ratio (HAR) etching and we elaborate on the methods of PE-ALE and PE-ALD as surface-controlled processing, as opposed to the conventional flux-controlled processing such as RIE and chemical vapor deposition (CVD). Second, we discuss the surface reaction mechanisms of PE-ALE and PE-ALD and the roles played by incident ions and radicals in their reactions. More specifically, we discuss the role of transport of ions and radicals, including their surface reaction probabilities and ion-energy-dependent threshold effects in processing over HAR features such as deep holes and trenches.