The different paradigms on which traditional information retrieval systems are built are reused to design Web search engines. However, designers have to innovate in order to adapt to this particular documentary context, which has an important impact on the way tools operate. Through an analysis of French and English scientific literature, and observations made during Web search training sessions, we try to define how far users are taken into account in the design of such systems, by comparing paradigm implementation to recent researches on information needs and the informational process. An analysis of the different steps of an information retrieval process highlights the complexity and the many dimensions of the concept of relevance in document retrieval.