Regardless of whether an organism has a large or a small genome, the genomes of all organisms are large relative to the size of the cell. For example, if the circular genome of the intestinal bacterium Escherichia coli were fully extended, its length would be 200 times greater than the diameter of the cell itself. The fully extended length of DNA in human chromosome 1, our longest chromosome, would be 10,000 times greater than the diameter of the average human cell. There is consequently a need to package an enormous length of DNA into a form that will fit inside the cell while still allowing the DNA to replicate and carry out its coding functions. The mechanism of packaging differs substantially in bacteria, archaeons, and eukaryotes. We focus here on bacteria and eukaryotes, primarily because less is known about how DNA is packaged in archaeons.