Copy detection has very important application in both intellectual property protection and information retrieval. Currently, copy detection concentrates on document copy detection mainly. In early days, document copy detection concentrated on program plagiarism detection mainly and now the most studies are on text copy detection. In this paper, a comprehensive survey on natural language text copy detection is given, the developments of copy detection is introduced. The approaches and features of a variety of existing text copy detection systems or prototypes are reviewed in detail. Then some key detection techniques are listed and compared with each other. In the end, the future trend of text copy detection is discussed.