How do experts spot masterpieces? Paintings are not always signed or noted in historical records, so how can we tell an obscure gem from an altered image? Scientists, conservators and art historians use a range of methods to examine the physical nature of pictures and unravel their hidden histories. Through a series of intriguing examples and clearly explained processes, this new addition to the National Gallery's popular Closer Look series will draw the reader into the complex issues-not all of them fully resolved-confronted by gallery professionals.