Peripheral data-storage units based on magnetic recording play a major role in computer systems. They have access times ranging from tens of milliseconds to seconds, contrasting with the microsecond or submicrosecond performance of core or semiconductor main-frame stores, but provide an online storage capacity of millions of characters at costs of ‘millipence’ per bit. The major components are the magnetic-recording or storage medium, the recording head, the transport mechanism and the electronics. The transport mechanism moves the heads and medium accurately with respect to each other; the control electronics control the mechanism in response to computer command; and the data-channel electronics, acting through the recording heads, write and read data to and from the storage medium. In the paper, the functions and general performance parameters of these units are reviewed, and the basic magnetic-recording principles are explained, with emphasis on the design of the recording medium and heads. Exchangeable-disc stores and magnetic-tape transports are then described in some detail, and attention is drawn to the level of development in many closely related branches of engineering that makes the present overall performance possible. Other magnetic peripheral stores such as drum and magnetic-card files are briefly reviewed, and the paper concludes by considering the limits of this technology and briefly discussing possible future technologies in this area.