Partition or distribution coefficients (and increasingly referred to as partition ratios) are widely used in environmental science to relate the concentration of a chemical solute in one phase to that in a second phase between which equilibrium applies or is approached. The solutes include organic and inorganic substances; the focus of this paper being on the former. The phases of interest include air, water, soils, sediments, aerosols, and biotic phases, such as lipids, blood, and various tissues. Availability of reliable partition coefficients for contaminants is essential for regulatory and scientific purposes, the general aim being to understand and predict the distribution of the substances in multimedia environmental and biological systems. The history of partition coefficients is reviewed, followed by a brief outline of their theoretical basis and a discussion of methods for determining partition coefficients both empirically and using a variety of predictive methods. It is suggested that ultimately a combination of empirical measurements, quantitative structure–property relationships, and computationally intensive quantum chemical molecular modeling techniques is required to provide accurate data for the large and increasing number of chemicals of commerce that may enter the environment.