The development of molecular physics and physical chemistry cannot be understood without a knowledge of the work of the Dutch physicist, Johannes Diderik van der Waals. His work is the foundation of much modern work on the theory of fluids. His doctoral thesis of 1873 was the first theory of liquids and gases in which the essential differences and similarities of these two phases were interpreted in terms of the properties of the constituent molecules. In the years since the end of the Second World War, there has been a re-appraisal of the importance of van der Waals's work, which has put van der Waals into his historic place as one of the founders of molecular science.