This monograph has been written from our convic tion that the present notions of the state of wa ter in osmotic systems are obscure, if not incor rect. The basic ideas presented herein are for us not original, but they have previously been ig nored. We shall attempt again to bring the essen tial concepts to the attention of the functional biologist with the hope that they will be duly considered and accepted. We even dare to expect that many will be able to recognize the inherent beauty in the old idea that all colligative pro perties of water stem exclusively from the fact that the water is under tension, regardless of the particular process by which it has been placed under tension in the biological system. The ideas are so simple and so obvious that we are amazed biologists have not already recognized their va lidity even without the assistance of physical chemistry and chemical thermodynamics, which deal with the subject of water under pressure in solu tions and in matrices. We expect that drawing at tention to experimental evidence heretofore not available will assist the more conservative phys iologist to reconsider and reject fictitious no tions about the properties of water in a solution.