The present paper is set in the field of discourse-oriented Neurolinguistics. A gradual classification of prepositions (in contrast with the traditional categorical ones) offered interesting possibilities when describing the behaviour of prepositions in the speech of subjects with a heterogeneous language. In these cases, the currently available classifications of prepositions were not considered adequate. The reasons why another classification (in which the prepositions are put on a scale according to their degrees of grammaticalization) is preferred will be given after a review of the works published on categories of prepositions. Classifications suggested by grammarians and neurolinguists concerned with describing the agrammatical speech of subjects with production aphasia will be examined in order to defend a gradual classification of prepositions.