The old master paintings we see today do not correspond to what the artist saw. Pictures change over time, often dramatically. This book unravels this phenomena for Jheronimus Bosch. It provides fundamentally new insights into the making of his paintings and into what has changed since then, which has significant consequences for their interpretation. This publication also compares how conservators deal with altered appearances. Strikingly, conservators determine how a painting can be seen and interpreted, by removing old ambiguities and by uncovering and unifying original elements. Even treatment of the reverse, the panel support, shows to impact the painted side. Contrary to common belief however, conservators do not restore pictures to their former glory. What does this mean for Bosch’s paintings?