This edited volume deals with the state of water in the vicinity of biological interfaces, both intracellular and extracellular. This issue is of critical importance, for the cell is extremely crowded with interfaces, and as a result practically all cell water is interfacial. The character, or state, of this water may therefore be central to cell function. What is meant by the ‘state of water?’ Few would question that water coming out of a household tap is a liquid, but water in an ice cube is something altogether different: it is a solid that floats on tap water (also known as bulk water). It is water in the solid state. The fact that ice floats is an indication that it is less dense than water. Clearly, the physical properties are different. Water molecules below 0 C form a crystal. In this crystal, the two positively charged hydrogen atoms of water bind to the double negative charges of oxygen atoms of two adjacent water molecules. The resulting crystal lattice is arranged in such a way as to be less dense than tap water, and constituent water molecules are also less mobile.