In this paper, we present two methods for speech understanding : an artificial neural network and an information theory based method. For both methods we have to index input sentences using semantic classes (or concepts). In the first method, we perform supervised learning and we obtain very good indexing results. In the second one, we propose a new method based on mutual information statistical measure to retrieve concepts, and also to tag each sentence by its concepts. Both methods have been tested on a tourist information corpus. The information theory method yields better recall, whereas the neural network achieves a better precision. Better performance has been obtained by the neural network method (about 4%).