High level ab initio methods have been used to study stacking interactions in standard A- and B-DNA, as well as in selected B-type crystal structures. A combination of quantum mechanical and classical mechanical methods has been used to examine cooperative contributions to stacking in DNA polymers. The results provide valuable information on intrastrand, interstrand, and cooperative contributions to stacking in DNA, and are useful for understanding the role of stacking interactions in the stabilization of DNA structures.