A client system (200, 220) receives a content item. The user can mark the received content item as being of interest. In response to said marking, tracking means (204, 224) automatically provide identifying data for the marked content item to a remote server system (250). The server receives a portion of the content item from a client system (200, 220), processes the received portion to obtain an identifier for the content item, obtains further information on the content item using the identifier, and transmits the further information back to the client system (200, 220). The tracking means (204, 224) subsequently receive further information on the content item from the remote server system (250). An identifier for the content item can be used in an e-commerce system (160), for example by putting it on a shopping list (161) or to obtain a list of related items.