The first well-known theoretical work on DNA denaturation by heating (without solvents) called the Poland-Scheraga model, discussed by C.Richard and A.Guttmann. According to the authors: "The question of the mechanisms applied to real DNA responsible for the denaturation process and which explain behavior as observed in melting curves is still far from being satisfactorily answered." The efforts toward improvement and further development or simplification of this model continue. Another rapidly developing direction of theoretical work toward understanding the denaturation process is the Nearest-Neighbor method, where the DNA helix is treated by theory with experiment (melting curves for short segment DNAs) indicates that the base pair sequence has a relatively small influence on the stacking free energy. The agreement between theory and experiment is obtained for equilibrium transitions of 14 out of 15 fragments 80-587 bp (base pairs) long. The deviation between theory and experiment for a 516 bp DNA can be attributed to the formation of stem-loop structures. The explanation of inconsistent results observed with long DNA fragments was also attributed to the possible formation of loops.