The concept of reverse Turing tests, or more commonly known as CAPTCHAs, for distinguishing between humans and computers has been around for many years. The widespread use of CAPTCHAs these days has made them an integral part of the internet for providing online services, which are intended for humans, with some level of protection against automated abuse. Since their inception, much research has focused on investigating various issues surrounding the design and security of CAPTCHAs. A fundamental requirement of CAPTCHAs necessitates that they must be designed to be easy for humans but difficult for computers. However, it is well recognized that the trade-off between usability and security is difficult to balance. In addition, numerous attacks have been developed to defeat CAPTCHAs. In response to this, many different CAPTCHA design variants have been proposed over the years. Despite the fact that CAPTCHAs have been around formore than two decades, the future of CAPTCHAs remains an open question. This chapter presents an overview of research examining a wide range of issues that have been conducted on different types of CAPTCHAs.