The aim of this masters thesis is to examine query expansion. Query expansion is the process of adding new terms to a query to improve the retrieval effectiveness. In this study the baseline query was expanded in five different modes. The first expansion strategy was formulated by the inflected terms as the system does not allow truncation, I have inflected the terms, e. g. airplane, airplanes, the airplanes, etc. in the baseline strategy. The second expansion strategy consisted of synonyms from a dictionary of synonyms. For the terms in the remaining three expansion strategies a general thesaurus was used which contained hierarchical and associative term relationships in order to find broader, narrower and related expansion terms. Relative recall and average precision was measured. The average results show that the expansion with inflected terms from the baseline query gave the most effective retrieval, both concerning increase in relative recall and average precision.