The structure of liquid water and its interaction with biological molecules is a very active area of experimental and theoretical research. The chemically complex surfaces of protein molecules alter the structure of the surrounding layer of hydrating water molecules. The dynamics of hydration water can be detected by a series of experimental techniques, which show that hydration waters typically have slower correlation times than water in bulk. Specific water-mediated interactions in protein complexes have been studied in detail, and these interactions have been incorporated into potential energy functions for protein folding and design. The subtle changes in the structure of hydration water have been investigated by theoretical studies.