A review is presented concerning the evolution of the fugacity concept as applied to environmental science. The series of serendipitous events that ultimately resulted in publication of the paper “Finding Fugacity Feasible” in 1979 is described. The use of fugacity as a surrogate for concentration is shown to facilitate the compilation and solution of mass-balance equations. It has proved to be valuable in a number of contexts, notably the description of chemical fate in unit worlds at various levels of complexity. More complex systems can be simulated as sets of connected unit worlds. The fugacity approach enables the multimedia character of organic chemicals to be deduced, thus contributing to the evaluation of chemical properties that impact persistence and long-range transport. It has proved to be especially insightful for describing bioconcentration, bioaccumulation, and pharmacokinetic phenomena. Applications to the sensing and monitoring of chemical presence in the environment are described. Suggestions are made for subject areas in which the fugacity concept may prove to be particularly valuable in the future. Finally, the many colleagues who have contributed to the use of fugacity when quantifying chemical behavior in the environment are acknowledged.