The Internet and World Wide Web have added just another medium for fast access to large amounts of information. It can save researchers lots of time in retrieving the required information and allows access to unexplored data which are worth many research projects in their own right. However, the flood of information on the web has become so large that now, when searching for a given piece of information, we are about to spend more time in browsing the web than we used to need searching in the library a decade ago (when the amount of available information was substantially less).
In the early years of networking we were happy when we could get electronic copies of astronomical catalogues without the delays through shipments of tapes. Now we are so flooded with them that in the rush of using many of them at the same time we sometimes forget that each one of them is telling us a different story. We must still read their detailed documentation if we want to derive reliable results from the available data. We have gone a good part of the way already to the point where all past issues of the major astronomical journals will be available electronically on the web. However, network saturation still keeps us from skimming a journal in the way that we could in the library.
A compromise has yet to be found between a rigorous refereeing system of web pages (as proposed by some) and the absolute liberty we currently ``enjoy'' in offering our own information and expressing our interests on the web. Many people have tried in recent years to offer guides to certain parts of astrophysical information, and the present article is just another example. The challenge for the future is how to protect ourselves from too much redundant or superseded information. While preparing this paper I came across many web pages which at first sight looked promisingly complete. However, when the last update (if given at all) was more than about a year ago, I usually refrained from quoting it here, because of the danger that it would not be maintained any more, or that it would offer too many outdated links. Perhaps a step towards reducing this danger could be a web browser that automatically recognized the date of latest update of a web document and would allow to set filters on that date in a search for relevant links. Actually, the AltaVista search engine (www.altavista.digital.com) allows a range of ``last modified'' dates to be used in advanced searches.
I am grateful to the School organizers for the opportunity to give these lectures and for their financial support. My special thanks go to Clive Davenhall, Elizabeth Griffin, and André Fletcher for their careful reading of the manuscript. Useful information or comments on the text were provided by P. Boyce, P. Eenens, G. Eichhorn, D. Fullagar, G. Giovannini, D. Golombek, C. S. Grant, U. Grothkopf, R.J. Hanisch, M. Irwin, F. Kupka, S. Kurtz, N. Loiseau, A. Macdonald, F. Murtagh, F. Ochsenbein, G. Paturel, M. Schmitz, S. A. Trushkin, M. Tsvetkov, and M. J. West. My apologies to those I forgot to mention here.