A banknote acceptor for authenticating a banknote 2, has a sensing station S with detector 18 that produces image signals corresponding to a face the banknote. A processor 24 pre-processes the image signals and compares them with stored reference data corresponding to an acceptable banknote from memory 25 to determining the authenticity of the banknote and its denomination. A display device 29 displays an image 31 corresponding to the banknote under test based on the image signals from the detector 18, to allow the user to perform a visual check that the banknote detected at the sensing station corresponds to the banknote under test presented by the user. Also, the detected denomination of the banknote is displayed on device 32. The user can accept or reject the banknote using buttons 33 and 34, and retrieve images of previously accepted banknotes with button 35.