The 'Diccionario del español medieval' (DEM) records and describes the lexicon of the Spanish language from the first documents up to about 1400. It will comprise not only the vocabulary of the literary works, but also that of texts insufficiently analyzed in traditional lexicography (collections of charters, Fueros, legal documents, translations of the Bible, chronicles, treatises, scientific works, etc.). It is based on more than 500 works,text collections and texts of the above-mentioned period as well as on more than 200 vocabularies, glossaries, concordances and lexical studies.The data of etymological research in the field of Romance linguistics are also taken into consideration. Each article of the dictionary consists of three parts: firstly, the documentation, which offers, along with the inventory of the first appearances of the word, numerous examples of its use on a chronological and semantic basis; secondly, the lexicographical information, which complements the documentation by adding further important items extracted from old Spanish dictionaries up to the "Diccionario de Autoridades" (1726-1739); and finally the lexicological and linguistic interpretation of the lexical material. Conceived in this way, the DEM combines the characteristics of a dictionary covering a period of the language with those of an etymological and historical dictionary.