Scholars have long recognized the importance of Paul’s citations from the Pentateuch for understanding the argument of Galatians. But what has not been fully appreciated is the key role that Isaiah plays in shaping what Paul says and how he says it, even though he cites Isaiah explicitly only once (Isaiah 54:1 in Galatians 4:27). Using an intertextual approach to trace more subtle appropriations of Scripture (i.e., allusions, echoes and thematic parallels), Harmon argues that Isaiah 49-54 in particular has shaped the structure of Paul’s argument and the content of his theological reflection in Galatians. Each example of Isaianic influence is situated within its original context as well as its new context in Galatians. Attention is also paid to how those same Isaianic texts were interpreted in Second Temple Judaism, providing the larger interpretive context within which Paul read Scripture. The result is fresh light shed on Paul’s self-understanding as an apostle to the Gentiles, the content of his gospel message, his reading of the Abraham story and the larger structure of Galatians.