This book discusses the CAPTCHA (completely automated public Turing test to tell computers and humans apart), an artificial-intelligence-based test commonly used on websites for preventing security attacks, covering its foundations, the main trends and challenges, as well as innovative features and research aspects. It focuses on the recently proposed CAPTCHAs, introducing research approaches for analyzing of the usability of the CAPTCHA test for Internet users. The book enables readers to become familiar with the CAPTCHA technology from the user's perspective, from the basic concepts underlying the test to more advanced concepts. It is intended for students as well as graduates wanting to gain insights into new CAPTCHA designs as well as new research directions.