Though quantum theory is celebrating its 100th anniversary this year, quan tum information processing is still a remarkably young research field. The questions driving this research field reflect a profound change in the gen eral attitude towards the fundamental aspects of quantum theory. So far, research on the foundations of quantum theory has been concerned mainly with the theoretical exploration of those particular features which distin guish quantum theory from classical physics. The main intention of quantum information p~ocessing is to exploit these specific features for technological purposes. As early as 1935, Erwin SchrSdinger had already noted that one of these characteristic features of quantum theory is the phenomenon of entan glement. Many years passed from this early insight until John Bell realized the quantitative consequences of the corresponding quantum correlations in his famous work from 1964. These theoretical predictions inspired numerous experiments, which all support the peculiar features predicted for quantum correlations. From these purely theoretical insights, it again required a long period of development to arrive at those potentially useful applications which are now of central interest for the processing of quantum information. The following contributions provide an introductory overview of basic problems, methods and topical results in this research field. The idea of pro ducing this volume was born at a symposium on this subject which was held at the 1999 annual spring meeting of the Deutsche Physikalische Gesellschaff in Heidelberg.
