In the course of his distinguished career of over 55 years, Kenneth S Pitzer published over 360 scientific papers. Included in this volume are 72 papers, selected for their historical importance and continuing significance. In early work, where spectroscopic data were incomplete or, later on, where the systems of interest were so complex that a deductive solution from molecular information was impractical, Pitzer interrelated molecular structural information, statistical methods and thermodynamic measurements to advance the understanding of molecular systems. This volume considers all three aspects and, by putting together selected papers, highlights the cohesiveness of certain advances through time and development. Several papers from journals not widely circulated can also be found in this selection of papers.