Topoisomerases are ubiquitous proteins found in all three domains of life. They change the topology of DNA via transient breaks on either one or two of the DNA strands to allow passage of another single or double DNA strand through the break. Topoisomerases are classified into two types: type I enzymes cleave one DNA strand and pass either one or two DNA strands through the break before resealing it, while type II molecules cleave both DNA strands in concert and pass another double strand through the break followed by religation of the double strand break. Here we review recent work on the structure of type I enzymes. These structural studies are providing atomic details that, together with the existing wealth of biochemical and biophysical data, are bringing our understanding of the mechanism of action of these enzymes to the atomic level.