This dissertation investigates the structure of the Spanish nominal phrase. First of all we have to discuss whether the noun really is the head of the phrase, as traditional grammars assume, or whether it is the determiner that constitutes the head of a determiner phrase. In order to resolve this problem we have to take a close look at the interrelation between nouns, determiners and adjectives. Spanish grammars mostly do not distinguish between adjectives and determiners. Both parts of speech are considered adjectives. We need a differentiation that includes the article, which is traditionally supposed to be outside. The nominal phrase must not consist of various parts, only a noun is possible. If determiner and adjective are not obligatory, the question of their status remains open, especially in a grammatical theory like HPSG. There are some arguments in favor of their status as complements, others in favour of their status as adjuncts, others for an intermediate status. These facts require a special treatment by the grammar. The differentation between determiners and adjectives demands special features, which include morphological, syntactic and semantic properties. Some special treatment is needed for the cooccurrence of determiners. Because this differentiation is gradual, we calculate the distance between one determiner and the other with the aid of a mathematical method. As a result the gradation is illustrated in a dendrogram. The HPSGrammar must be modified by an idiosyncratic sort of hierarchy for Spanish, so that the parts of speech are adequately accommodated. We can demonstrate that the HPSGrammar is able to integrate the special requirements of the Spanish determiner system, in which the article can be included. The nominal phrase is governed by the noun. Determiners are classed as specifiers between complementation and adjunction, while adjectives are classed as adjuncts.