This article sets out the sources, both lexicographical and other, used to create the Nuevo Tesoro Lexicográfico del Español (s. XIV–1726). Among these, particular attention is due to the pre-1500 sources, especially the works of Antonio de Nebrija (1444?–1522), because of their importance for subsequent lexicography, including the pioneers of bilingual lexicography who set Spanish alongside another language. Information is also supplied about the number of works and records consulted, the difficulties arising from treating the data, and the method adopted for grouping together variants of the same form. Finally, an indication is given of the interest of the NTLE for the history of lexicography.