Pulsed-field gel electrophoresis (PFGE) is a technique for resolving large (up to 10 Mb) DNA molecules. Using multiple pairs of electrodes DNA is subject to an alternating electric field through a solid agarose matrix. As the current changes direction the reorientation time of DNA is proportional to molecular weight; thus fragments are separated in the gel based on their size. Here we describe the use of PFGE to analyze DNA double-strand break formation and repair in human chromosomal DNA.