In the plasma etching process it is important to determine when the endpoint occurs for a process step, in order to precisely control the process with optimal throughput. This entry reviews literature on the various methods of endpoint detection in plasma etching. A variety of techniques have been published since the debut of the plasma etching technology. This area of research was very active from 1970s to 1990s, and the advantages and weaknesses of these techniques were analyzed and tested by many researchers. Since the mid-1990s, OES (Optical Emission Spectroscopy) has become the dominant technique for endpoint detection in the industry because of its good performance, reasonable cost, and compatibility to the manufacturing environment. With the emergence of advanced device structures and new materials as well as the requirement for more precise process control due to further scaling down of device dimensions, more innovative endpoint detection techniques need to be explored.