The invention relates to a printing apparatus comprising a printing track defining a print receiving surface and a printing carriage arranged to travel along the printing track. The carriage includes printing heads arranged to receive data from a central database for printing on the printing surface at predetermined positions along the printing track, and a roller arrangement for driving the carriage along the printing track. An optical position encoder is provided for monitoring the movement of the carriage as it travels along the printing track. A data transmission link is provided for interfacing the central database to the carriage. The central database includes both positional data for controlling the position of the printing heads and pricing data. The printing track has mounting clips for mounting it on a shelf edge, and the printing heads are arranged to label items of stock on the shelf. The invention extends to the printing track, the printing carriage, a shelf labelling system, as well as to a method of labelling a row of items of stock.