In the semiconductors and related industries, the fabrication of nanostructures and nanopatts has become progressive demand for achieving near-atomic accuracy and selectivity in etching different materials, particu-larly in ultra-thin gate dielectrics and ultra-thin channels used in field-effect transistors and other nanodevices below 10 nm scale. Atomic layer etching (ALE) is a novel technique for removing thin layers of material using sequential and self-limiting reactions. Different from most ALE processes using plasma-enhanced or other en-ergetic particles-enhanced surface reactions, thermal ALE realizes isotropic atomic-level etch control based on sequential thermal-drive reaction steps that are self-terminating and self-saturating. Thermal ALE can be viewed as the reverse of atomic layer deposition (ALD), both of which define the atomic layer removal and growth steps required for advanced semiconductor fabrication. In this review, we focus on the concept and basic character-istics of the thermal ALE in comparison with ALD. Several typical thermal ALE mechanisms including fluor-ination and ligand-exchange, conversion-etch, oxidation and fluorination reactions are intensively introduced. The pros and cons of thermal ALE, plasma ALE, and traditional plasma etching are compared. Some re-presentative materials and their typical thermal ALE processes are summarized. Finally, the outlook and chal-lenges of thermal ALE are addressed.