Since the introduction of optical data storage systems in the 1970s, we have observed a stepwise increase in their storage capacity using the same means for resolution improvement as in classical microscopy and optical lithography, namely, a reduction in the source wavelength and an increase in the numerical aperture of the imaging optics. In this paper we briefly address the historical development of optical data storage and some recent developments towards higher density such as non-linear recording methods and systems with a numerical aperture larger than unity. More specifically, we explore the possibility of storing more information bits per storage location so that optical 'multiplexing' becomes feasible. A multiplexing method based on the detection of optical angular momentum of a focused light beam is treated in detail and is illustrated with some examples of preliminary experiments on this subject. Both the existing high-density systems and the proposed new ones require a detailed analysis of the focusing of the scanning spot and the diffraction by the information structure on the disc. We analyse electromagnetic focusing in multilayers and treat the diffraction of light by optical effects using a three-dimensional form of Green's tensor formalism.