Small DNA circles can occur in Nature, for example as protein-constrained loops, and can be synthesized by a number of methods. Such small circles provide tractable systems for the study of the structure, thermodynamics and molecular dynamics of closed-circular DNA. In the present article, we review the occurrence and synthesis of small DNA circles, and examine their utility in studying the properties of DNA and DNA–protein interactions. In particular, we highlight the analysis of small circles using atomistic simulations.