Water is well known to be an essential component of all biological systems. Thousands of papers describe a role for water interacting with ions, proteins, nucleic acids, and lipids. In this book, the authors discuss the background chemistry and physics, including a very brief review of thermodynamics and quantum mechanics, to the limited extent needed for the remainder of the book. The various behaviors of water at surfaces, in solutions, in hydrating ions, proton transport, as well as the phase changes that involve water, are introduced. Hydrogen bonds, their networks, and the way they are associated with proteins are discussed, along with proton transport, which is important in a number of biological systems. The authors then proceed to discuss water in confined spaces, and in channels and transporters, which are topics particularly important in biology. Some special topics are interesting, like anhydrobiosis, and those are touched upon briefly. The authors have made no attempt to make this book comprehensive; the thousands of papers that have been published that are relevant provide far too much material for a single volume. Enough material is presented so that the reader sees what is now known, or at least is thought to be known. As with any science, sometimes we are pretty sure of something, only to find ground shifting under us as new experimental results show that what we thought we knew, we didn’t. However, a snapshot of the present state of knowledge is useful. Knowledge is progressing so rapidly that at least some will need updating as this is being written, but this is a good time to summarize what is currently known, and provide background information that will help understand what is coming.