Shadows are holes in light. They are see all the time and sometimes noticed, but their part in our visual experience of the world is mysterious. This text draws on contemporary cognitive science, 18th-century theories of visual perception and art history to discuss shadows and the visual knowledge they can offer. The author begins by describing the physical constitution and different varieties of shadows. He then sketches the 18th-century empirical/nativist debate on the role of shadows in the perception of shape. Next, he surveys modern research by cognitive scientists and machine vision workers, explaining how research is divided on the issue of how far and by what means shadows help or hinder perception of shape. The book continues the exploration by recounting a neglected episode of shadow theory, the observations of a group of mid-18th-century French scientists and artists on shadows as related to light and space. Finally he sets these various shadow universes into relation with each other, addressing the special problem of painting shadows and analyses Chardin's painting, "The Young Draughtsman", in which shadow painting is both medium and theme. The book includes an appendix that situates and summarizes the shadow system of Leonardo da Vinci, which has had a strong - though partly underground influence on thinking about shadows - for 500 years.