Preliminary drawings beneath the surface layers of a painting can often be made visible by a technique called infra-red reflectography. The fascinating revelations of such underdrawings are always a source of important material for art history, leading to deductions about attribution, style, changes in composition, and workshop practices. This accessible book presents the results of pioneering research on the underdrawings of fifteenth- and sixteenth-century paintings by conservation, scientific, and curatorial staff at the National Gallery in London.