Differential Object Marking (DOM) in Spanish has been studied under various aspects: Spanish as a typical DOM language in typology, Spanish as the most salient Romance DOM language, Spanish and its diachronic evolution and diatopic variation. The present paper discusses different accounts that were given in the literature for the emergence and tendencies of DOM in Spanish. It tries to show that there is, apart from all the semantic and syntactic factors traditionally discussed, an overall basic value of DOM in Spanish which can be extracted analysing those cases where the presence or absence of DOM establishes an obvious opposition in meaning. It is claimed that the contemporary system emerged in Early Modern Spanish and that it seems rather stable in the Spanish speaking world. Furthermore, as a contemporary tendency within this system there seems to be an increase of DOM in cases where marking is facultative, with regional differences within the Spanish speaking world.