How do dealers, auctioneers and museum staff determine whether a piece is by one painter or another? What role does the signature play, and what are the key clues to look for when deciding who painted the picture? Artists before the 18th century usually worked with a number of assistants around them, who were 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.