DNA double-strand break repair is a complex process that requires multiple enzymatic and structural activities to rejoin or repair the broken DNA ends using one of several repair pathways. These enzymatic and structural activities include end detection, end processing and alignment of DNA ends. Recent structural and functional studies of the DNA double-strand break repair factors Mre11/Rad50, Ku70/80 and Xrcc4 show how these enzymes combine and assemble both enzymatic and structural activities in DNA double-strand break repair.