We review our recent efforts in understanding the elastic properties of double-stranded (ds) and single-stranded (ss) DNA macromolecules. A simple geometric model of dsDNA was constructed and solved by path integral methods. The good agreement with experiments on dsDNA's entropic elasticity, cooperative extensibility, and supercoiling property suggested that the short-ranged base-pair stacking interaction is crucial for the stability and the high deformability of dsDNA. For ssDNA at high ionic conditions, base-pairing and base-pair stacking interactions cause the polymer to fold into compact hairpin configurations. The force-induced hairpin-coil transition was studied with the generating function method. In accordance with experiment, this transition was found to be highly cooperative when the average stacking potential is higher than some critical level. At low ionic conditions, the electrostatic repulsive interaction along the ssDNA becomes dominant, and ssDNA can be regarded as model polyelectrolytes. Our MC simulation results suggested an exponential relation between external force and the total extension. This prediction was confirmed experimentally.