Art restorations often give rise to controversy, and the reason does not always seem to be a lack of skill or dedication on the side of the restorer. Rather, in some of the most famous cases, the reason seems to be a lack of agreement on basic principles. In particular, there seems to be a lack of agreement on how the following two questions are to be answered. First, what is art restoration supposed to achieve, in other words, what is the goal of restoration? Second, how can this goal be achieved, in other words, what are the means that a restorer can legitimately make use of? In this paper, I formulate, and defend, a principle that answers these questions. Although the principle may sound platitudinous, it has been contradicted by prominent philosophers and by important organizations concerned with conservation. Moreover, defending the principle turns out to be impossible without touching on such vexed philosophical issues as the ontology and identity of artworks, as well as the nature of perceptual and aesthetic properties (including the nature of artistic value).