When the Wallace Collection was bequeathed to the British nation in 1897 it was believed that the museum had twelve paintings by Rembrandt. Later, however, most of these pictures were re-assigned to other artists, to the extent that a hundred years later it was thought that there was only one genuine work by the great Dutch master - a wonderful portrait of the artist’s son Titus. Since then further research has suggested that in fact four of the other paintings are also wholly or partly by Rembrandt. But who decides things like this, and how do they make their decisions? Through telling this fascinating story we'll see how connoisseurship changes over time and the enormous impact that this can have on how we look at works of art.