The NEBNext Ultra II End Repair/dA-Tailing Module has been optimized to convert 500 pg-1 μg of fragmented DNA to repaired DNA having 5′ phosphorylated, 3′ dA-tailed ends. The module is optimized for use with the NEBNext Ultra II Ligation Module (NEB #E7595), and is part of the Ultra II DNA workflow which enables high yield preparation of high quality libraries from 500 pg to 1 µg of input DNA.