The paintings we see today in museums, galleries, churches and temples are often much altered by the centuries. Pictures can split, rot, be eaten by woodworm, warp, blister, crack, cup, flake, darken, blanch, discolour, become too translucent and disappear under a centuries-old varnish; and they can also suffer from the efforts of their owners to rectify these situations: they might be transferred, relined, ironed, abraded or repainted. Anyone considering a work of art needs to establish at the outset how much it has changed since it was first made. This act of understanding is far from easy. We need to develop a knowledge of the physical and chemical processes which have brought paintings to their current state, in the hope that we can imagine their reversal. And we have to look as much as we can at a wide variety of paintings, so we can learn to distinguish those in a worse or better state of preservation; we have to try to understand what it is about a picture that differentiates good and bad condition. Theories of art history have been built on works whose appearance is made up of little more than repaint and decay, and the beginner needs to be warned about the many pitfalls dug by time for the unwary. This book is meant both for that beginner and for the qualified practitioner who might have missed a step along the way.