Type II topoisomerases are ubiquitous enzymes that play essential roles in a number of fundamental DNA processes. They regulate DNA under- and overwinding, and resolve knots and tangles in the genetic material by passing an intact double helix through a transient double-stranded break that they generate in a separate segment of DNA. Because type II topoisomerases generate DNA strand breaks as a requisite intermediate in their catalytic cycle, they have the potential to fragment the genome every time they function. Thus, while these enzymes are essential to the survival of proliferating cells, they also have significant genotoxic effects. This latter aspect of type II topoisomerase has been exploited for the development of several classes of anticancer drugs that are widely employed for the clinical treatment of human malignancies. However, considerable evidence indicates that these enzymes also trigger specific leukemic chromosomal translocations. In light of the impact, both positive and negative, of type II topoisomerases on human cells, it is important to understand how these enzymes function and how their actions can destabilize the genome. This article discusses both aspects of human type II topoisomerases.