We define visualization as the communication of information using graphical representations. Pictures have been used as a mechanism for communication since before the formalization of spoken and written language. A single picture can contain a wealth of information, and can be processed much more quickly than a comparable page of words. This is because image interpretation is performed in parallel within the human perceptual system, while the speed of text analysis is limited by the sequential process of reading. Pictures can also be independent of local language, as a graph or a map may be understood by a group of people with no common tongue.