Astronomical photographs contain an enormous amount of information. This presents extremely interesting problems when one wishes to produce digitised sky atlases, to archive the digitised material, to develop sophisticated devices to do the digitising, and to create software to procecc the vast amounts of data. All these activities are necessary to be able to do astronomy.

One such activity is the important, large-scale optical identification of objects which also emit radiation at other wavelengths. Other activities of the past decade include a multiplicity of surveys that have been made on galaxies and clusters of galaxies.

This book treats, in five sections, the existing and future surveys, their digitisation and their impact on astronomy. It is a reference for people in the field and for those who wish to engage in using or producing sky surveys.