The use of images in human communication is hardly new: our cave ancestors painted pictures on the walls of their caves and the use of maps and construction plans to transmit information almost certainly dates back to pre-Roman times. But the twentieth century has witnessed an unprecedented growth in the number, availability and importance of images in all walks of life. Images now play a crucial role in such diverse fields as medicine, journalism, advertising, design, education and entertainment. Technology, in the form of inventions such as photography and television, has played an important role in facilitating the capture and communication of image data. But the real engine of the image revolution has been the computer, bringing with it a range of techniques for capturing, processing, storing and transmitting digital images that would surely have surprised even pioneers like John Logie Baird. Computer involvement in imaging dates back to 1965, with Ivan Sutherland's Sketchpad project, which demonstrated the feasibility of automated image creation, manipulation, and storage, although the high cost of hardware limited its use until the middle of the years eighty. Once the computerized image became accessible, it soon penetrated into areas traditionally heavily dependent on images for communication, such as engineering, architecture and medicine. Photography libraries, art galleries and museums also began to see the advantages of making their collections available in electronic form. The creation of the World Wide Web in the early 1990s, which allows users to access data in a variety of media from anywhere on the planet, has provided a massive stimulus to the exploitation of digital images. The number of images available on the Web was recently estimated to be between 10 and 30 million figures in which some observers consider to be a significant underestimate.