DNA polymerase δ (Pol δ) is a high-fidelity polymerase that has a central role in replication from yeast to humans. We present the crystal structure of the catalytic subunit of yeast Pol δ in ternary complex with a template primer and an incoming nucleotide. The structure, determined at 2.0-Å resolution, catches the enzyme in the act of replication, revealing how the polymerase and exonuclease domains are juxtaposed relative to each other and how a correct nucleotide is selected and incorporated. The structure also reveals the 'sensing' interactions near the primer terminus, which signal a switch from the polymerizing to the editing mode. Taken together, the structure provides a chemical basis for the bulk of DNA synthesis in eukaryotic cells and a framework for understanding the effects of cancer-causing mutations in Pol δ.