How do dealers, auctioneers and museum staff determine whether a piece is by one painter or another? What role does the signature play? Artists before the 18th century usually worked with a number of assistants trained to reproduce the style of the master as closely as possible. How can we study these workshop productions, distinguishing between master and pupil, master and copyists? In this lecture, we consider clues, weigh up their relative usefulness, and learn some of the tricks of the connoisseur.