When scientists or engineers are looking for information in document collections, or on the web, they generally have a precise ob jec- tive in mind. Instead of looking for documents ”about a topic T ”, they rather try to answer specific needs such as finding the definition of a con- cept, finding results for a particular problem, checking whether an idea has already been tested, or comparing the scientific conclusions of two ar- ticles. One of a ob jective of this thesis is to build an indexing model which includes the decomposition of documents into fragments that will cor- respond to discourse elements (definition, hypothesis, method, findings, etc.). The division of documents into fragments should allow scientists to retrieve more pertinant information and to make queries more pre- cise. Each type of discourse element will be modeled by defining specific characteristics.