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It is no longer possible to imagine the internet without search engines. The growth of net contents, helped by the low communicative access barriers, makes the selection of relevant contents necessary. Search engines assume this selection and mediation function at the interface between public and individual communication. Their ability to reduce the complexity of the web and extend the horizon of the purely human search in many cases enables certain information to be accessed at all (Rieder, 2005: 29f.). They therefore perform a function similar to that of the classical gatekeepers (Machill et al., 2004: 322). In line with the key role that they play, search engines are regularly called upon, in the US for example by 84 percent of internet users; 56 percent of internet users even access them on a daily basis (Fallows, 2005). In contrast to the considerable social importance of search engines and the power that they wield, large gaps in research are evident. These gaps must be closed. This contribution aims to elaborate the central dimensions of searchengine research, gather together existing insights and make suggestions for future research.A hallmark of the search-engine market is the oligopoly of the three US search-engine operators Google, Yahoo and MSN which, from a global perspective, share the majority of the search-engine market. Within this triumvirate Google assumes the position of the market leader. In the US, within a year, Google succeeded in increasing its share of searches by 6 percent to just under half (49 percent) in December 2005, whereas Yahoo remained static at 21 percent, MSN rose by 3 percent to 11 percent and the other providers lost 2 percent to arrive at a figure of 19 percent (Nielsen NetRatings, 2006).

The existence of a European public sphere is disputed -not only in communication and media science. A common thesis in research is

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