The state of water in colloidal systems is a controversial subject. Discovery of the colloidal state by Graham (18) was followed by extensive studies by Van Bemmeln (66, 67, 68, 60) and Zsigmondy (73, 74). Thermodynamic studies by Katz (36) indicated significant changes in free energy and heat content in colloidal systems at low water content. Overton (51) concluded that part of the water in animal tissue did not have normal solvent properties. The work of Rosa (59) and Hooker (28) suggested a relationship between certain hydrophilic bio-colloids and cold resistance in plant tissue. Newton and Gortner (49) proposed a cryoscopic method for the measurement of hydrophilic colloids as a criterion of the state of water in living and non-living colloid systems. The apparently unusual state of water thus disclosed by a vast amount of data has been called *'bound'' water. It has been assumed that the presence of colloidal material was responsible for the observed phenomenon. It is of particular importance in biological systems because of certain implications with regard to hardiness and drought resistance in plants and animals. The majority of investigations have dealt with plant physiology. The estimation of bound water is based upon the physical measurement of a variety of properties of water. Some workers compute the amount of bound water from the total change in thermodynamic activity of the water in a given system while other workers include only that part of the change in thermodynamic activity which cannot be accounted for by osmotically active substances. In this paper the calculations are based upon the latter concept and bound water is denned as the amount of water required to compensate for the deviation of some thermodynamic property of water from the measurement expected in simple solution. It has been suggested (17) that bound water is due to the orientation of water molecules about the colloidal particles and that the water, thus removed from the body of the solution, loses its solvent properties. A few investigators have disagreed with these premises. The data of Hill (27) and Grollman (21) did not show appreciable amounts of bound water in many colloidal systems. Greenberg and Greenberg (20) subjected the concept of lost solvent powers to test and found no evidence for bound water on that basis. A knowledge of the nature of the phenomenon observed is necessary for the interpretation of its r?le in biological processes.