Until now, there has been no general theory of learner's dictionaries. This article takes steps towards such a theory based on the idea that lexicography is an independent discipline that needs to develop its own concepts, methods and partial theories. In this perspective, it presents first a proposal for a general definition of a learner's dictionary and then discusses the relation which from the point of view of lexicography exists between linguistic capacity, linguistic knowledge and communication. Finally, it presents a typologization of the potential users of a learner's dictionary based on their foreign language level and the different extra-lexicographic situation where they may have needs that can be resolved by means of dictionary consultation.