One of the most significant developments in the theory of solutions occurred in 1951, when Kirkwood and Buff 1 published their paper entitled “The Statistical Mechanical Theory of Solutions”. The Kirkwood-Buff (KB) formalism yields rigorous expressions for key thermodynamic derivatives (such as isothermal compressibility, partial molar volumes, and chemical potential compositional derivatives) in terms of microscopic quantities (correlations between pairwise concentration fluctuations, or, equivalently, integrals of pair correlation functions). Because of its theoretical foundation in the statistics of concentration fluctuations in the grand canonical ensemble, the formalism is also frequently referred to as the fluctuation theory of mixtures.