DNA looping is widely used in nature. It is well documented in the regulation of prokaryotic and eukaryotic gene expression, DNA replication, and site-specific DNA recombination. Undoubtedly looping also functions in other protein-DNA transactions such as repair and chromosome segregation. While the underlying physical chemistry of DNA looping is common to all systems, the precise biochemical details of looping and the utilization of looping by different systems varies widely. Looping appears to have been chosen by nature in such a wide variety of contexts because it solves problems both of binding and of geometry. The cooperativity inherent in binding a protein to multiple sites on DNA facilitates high occupancy of DNA sites by low concentrations of proteins. DNA looping permits a sizeable number of DNA-binding proteins to interact with one of their number, for example RNA polymerase. Finally, DNA looping may simplify evolution by not requiring a precise spacing between a protein's binding site and a second site on the DNA.