Various properties of the DNA molecule are comprehensively treated within the framework of highly simplified but very useful theoretical models. Some of these models, the results obtained and their comparison with experimental data will be considered in the talk. Perhaps the most successful of DNA models treats the molecule as a homogeneous and isotropic elastic rod. This model quantitatively explains the macromolecular behavior of linear DNA. It also provides a major clue to understand closed circular DNA molecules (ccDNA). The theoretical treatment of ccDNA heavily relies on the knot theory. Monte Carlo simulations of ccDNA make it possible to predict the equilibrium probability of knot formation in DNA as a function of its length and width, which quantitatively agrees with experiment. Other DNA theoretical models, the polyelectrolyte model and the helix-coil model, which make it possible to consider DNA behavior depending on the ambient conditions, will be discussed, if time permits.