Molecular thermodynamics is essentially the application of statistical mechanics to calculate thermodynamic and transport properties of fluids and materials. Compared to classical thermodynamics, which is general and macroscopic, molecular thermodynamics is based on an understanding of the structures and interactions at the molecular level, to model important quantities and phenomena in chemistry, engineering, environmental and life sciences.Major developments of recent years have been the advances in theoretical chemistry, which is necessary to describe intermolecular forces, and molecular simulation, which allows to study the collective behaviour of many thousands of particles. Some examples from an area that is active today, the field of ionic liquids, will illustrate the exploratory role of molecular simulation in the discovery of the fascinating properties of these new liquids.