We have previously found that the dominating force between DNA double helices once the interaxial spacing is less than about 35 angstroms is not any of the commonly considered forces, as electrostatic repulsion and van der Waals interaction. Rather, the force appears to be due to the structuring of water between the surfaces. This force between helices can either be strongly repulsive or strongly attractive depending on the structuring of the surface water. The discovery of this force profoundly affects the way one should think about the interactions of polar surfaces in the cell. Presently, we are determining the thermodynamics of the attractive force, the free energy, enthalpy and entropy of this interaction in order to define characteristics that can be used in understanding the interactions between other macromolecules. We are using the full power of the osmotic stress technique to map out phase transitions between repulsive and attractive hydration forces to determine the energetics of these forces. A systematic variation of ligand activity, temperature, and osmotic pressure enables us to dissect out the contributions to the energetics of these transitions from ligand binding and hydration energies. The most pertinent characteristic of attractive hydration forces between DNA helices we find is that it is an entropically driven assembly process, a characteristic shared by many other cellular assembly processes.