The Kirkwood-Buff Theory of Solutions: With Selected Applications to Solvation and Proteins presents the Kirkwood-Buff (KB) Theory of solution in a simple and didactic manner, making it understandable to those with minimal background in thermodynamics. Aside from the fact that the KB Theory may be the most important and useful theory of solutions, it is also the most general theory that can be applied to all possible solutions, including aqueous solutions of proteins and nucleic acids. Introductory chapters give readers grounding in the necessary chemical thermodynamics and statistical mechanics, but then move to a systematic derivation of Kirkwood-Buff theory and its inversion. Originally published in 1951, the KB theory was dormant for over 20 years. It became extremely useful after the publication of the "Inversion of the KB theory" by the author Arieh Ben-Naim in 1978. The book explains all necessary concepts in statistical mechanics featured in the theory in a simple and intuitive way. Researchers will find the theory useful in solving any problem in mixtures or solutions in any phase. Some examples of applications of the KB theory, to water, aqueous solutions, protein folding, and self-association of proteins, are provided in the book.