Abstract: Completely automated public turing test to tell computers and humans apart (CAPTCHA), also known as human interactive proof (HIP), is a program that can generate and grade tests that most humans can pass and current computer programs cannot. CAPTCHA provides a way for automatically distinguishing a human from a computer program. It has emerged as a standard network security technology and has been successfully used in many popular Web sites, such as Google, Yahoo!, and Microsoft. CAPTCHA is designed based on certain unsolved artificial intelligence (AI) puzzles and can be categorized into three main types, namely, text-based, image-based, and sound-based, according to its carrier and concept. Text-based CAPTCHA based on character recognition has been widely used. Image-based CAPTCHA that exploits harder problems in computer vision is still in the research stage. Sound-based CAPTCHA, which is aimed at providing access to the visually impaired, is a complement to visual CAPTCHAs. This paper introduces the history and the general principle of CAPTCHA. The latest research progress in the design of CAPTCHA is expounded by enumerating typical examples. Their usability and security are also discussed. Finally, the directions for future study as well as the problems to be solved are pointed out.