This review shows that water in biological systems is not just a passive liquid solvent but also a partner in the formation of the structure of proteins, nucleic acids and their complexes, thereby contributing to the stability and flexibility required for their proper function. Reciprocally, biological macromolecules affect the state of the water contacting them, so that it is only partly in the normal liquid state, being somewhat ordered when bound to macromolecules. While the compaction of globular proteins results from the reluctance of their hydrophobic groups to interact with water, the collagen superhelix is maintained by water forming a hydroxyproline-controlled frame around this coiled-coil macromolecule. As for DNA, its stability and rigidity are linked to water fixed by AT pairs in the minor groove: this leads to the enthalpic contribution of AT pairs exceeding that of GC pairs, but this is overbalanced by their greater entropy contribution, with the result that AT pairs melt at lower temperatures than GCs. Loss of this water drives transcription factor binding to the minor groove.