Authentication is probably one of the main security processes that almost everybody has at one point used. Currently, the most widespread authentication mechanism is based on textual passwords, a well-established approach that, with the growth of users and services, has increasing and serious drawbacks. With the rise of high quality displays and more ergonomic human computer interaction mechanisms such as mice, touch-pads and touch-screens, graphical passwords are credited as a valuable replacement to old-fashioned passwords. In contrast to alphanumerical passwords, graphical authentication mechanisms promise greater memorability and usability. In this chapter, an overview of the state-of-art of this topic is presented, introducing some of the main schemes proposed in current literature. The issues and concerns related to security and usability, which still challenge the researchers in this area, are also discussed.