Deuteronomy consists largely of the historical Moses’s final sermonic oracles, which he both spoke (Deut 1:3, 5; 4:44; 5:1; 27:11; 29:1[28:69]) and transcribed (31:9, 22, 24; 32:45) on the plains of Moab just prior to his death (32:48–52; 34:1–6). This study argues that the way Moses uses Scripture in this embedded material supports the view that the books of Genesis–Numbers were already in a preliminary canonical form just before the prophet’s death. The study will first synthesize the contemporary discussion of Deuteronomy’s potential sources, noting how scholars differ on which sources inform the book and what Deuteronomy’s relationship to them actually is. Next, the study engages in a series of case studies related to Deuteronomy’s use of antecedent narratives (related to the patriarchs and the revelation and rebellion at Horeb) and laws (like the Decalogue, the pilgrimage feasts, and the handling of skin diseases). It then considers the book’s hermeneutical and theological strategy, proposing that Deuteronomy uses Scripture to stress the lasting significance and certain fulfillment of past promises, to motivate loyalty by recalling past experience, and to indicate the abiding authority, implications, and fresh applications of past instruction. Finally, the study concludes by reflecting on the findings’ implications for the dating of a preliminary canonical Pentateuch. The conclusions support the view that Moses was the substantial author of Genesis–Deuteronomy, which affirms the Church’s long-standing pre-Enlightenment view yet counters the majority opinion today, even held by leading evangelicals like T. Desmond Alexander, whose influential introduction to the Pentateuch continues to place the shaping of the initial Pentateuch during the exilic period.