This volume contains the collected works of the eminent chemist and physicist Lars Onsager, one of the most influential scientists of the 20th Century. The volume includes Onsager's previously unpublished PhD thesis, a biography by H C Longuet-Higgins and M E Fisher, an autobiographical commentary, selected photographs, and a list of Onsager discussion remarks in print. Onsager's scientific achievements were characterized by deep insights into the natural sciences. His two best-known accomplishments are his reciprocal relations for irreversible processes, for which he received the 1968 Nobel Prize in Chemistry, and his explicit solution of the two-dimensional Ising model, a mathematical tour de force that created a sensation when it appeared. In addition, he made significant theoretical contributions to other fields, including electrolytes, colloids, superconductivity, turbulence, ice, electrons in metals, and dielectrics. In this volume, Onsager's contributions are divided into the following fields: irreversible processes; the Ising model; electrolytes; colloids; helium II and vortex quantization; off-diagonal long-range order and flux quantization; electrons in metal; turbulence; ion recombination; fluctuation theory; dielectrics; ice and water; biology; Mathieu functions. The different fields are evaluated by leading experts. The commentators are P W Anderson, R Askey, A Chorin, C Domb, R J Donnelly, W Ebeling, J-C Justice, H N W Lekkerkerker, P Mazur, H P McKean, J F Nagle, T Odijk, A B Pippard, G Stell, G H Weiss, and C N Yang.