We introduce a new method for automatically constructing concept hierarchies where the concept nodes follow a generalization / specialization relation. Starting from a set of concepts automatically extracted from a corpus, we show how to learn generalization / specialization relations between couples of concepts and how this leads to the construction of the hierarchy. We present an application of this method for building thematic document hierarchies similar in spirit to those found on internet portals. We also introduce new criteria for evaluating the quality of such hierarchies and for comparing them. We finally describe a series of tests performed on document collections coming from LookSmart and NewScientist hierarchies. KEYWORDS Structuring corpora, semantic relations between concepts, concept hierarchies, text segmentation