A tape cassette has a housing which contains at least one magnetic tape wound on a tape hub, and a semiconductor memory which contains memory data. The tape cassette also includes at least an optical transmitter, and a circuit which has access to the memory contents and which can read out the memory contents from the memory and embody the memory contents in an optical signal, which is transmitted out of the housing by the optical transmitter. The memory data may characterize data stored on the magnetic tape. The cassette can include a further memory which contains data uniquely identifying the cassette, and this data can also be read out and embodied in optical signals which are transmitted out of the housing. A number of such cassettes can be employed in a cassette library system, having a reader which receives the optical signals and which controls a robotic cassette selector by matching either the memory data or the cassette identification data embodied in the received optical signals to data entered into the reader identifying wanted data or a wanted cassette.