In this article, the analysis of the sources of Manuel de Larramendi’s (1690-1766) grammar and dictionary reveals the importance of his work and asserts that they are necessary tools to better understand the history of the Basque language. First, we will offer an overview of the reception of Larramendi’s works from the 18th century to the 20th (§ 1), in order to show that from the end of the 19th century onwards vascologists highlighted their apologetic aspect and downplayed their significance as the first printed Basque linguistic tools, just the opposite of what had happened previously. We will go on to evaluate the reliability and richness of his Diccionario Trilingüe (DT, 1745) with a sample (§ 2) to show the large number of words from the oral language collected by Larramendi, in what semantic fields they are concentrated and, ultimately, the relevance of his dictionary as a means of attesting words and variants, and dating them. As this paper presents the results of a first approach to the Basque sources of Larramendi’s grammar (1729), we will seek to establish that the Labourdin writers Etxeberri of Ziburu (1627, 1630) and Haranburu (1635) are some of them, and perhaps also Axular (1643), all of them known sources of his dictionary. Finally, regarding the Biscayan dialect, the data shows that, although some of the verb forms may have been taken from Capanaga's catechism (1656), others are coined by Larramendi analogically from the paradigms of the Guipuzcoan dialect.