Weak intermolecular interactions are not only important in modern supramolecular chemistry, they hold the organic world together and are responsible for the very existence of liquids and solids. A characteristic feature of weak interactions is the phenomenon of enthalpy-entropy compensation. Information about weak interactions in crystals is obtained from packing patterns, which provide tests for the quality of atom-atom force fields. On melting a solid, most of the cohesive energy is retained in the liquid state, even though the optimal mutual orientation of interacting parts of molecules is now opposed by thermal disorder. For liquids, reliable structural information is hard to come by, but extensive thermodynamic data are available for certain classes of compounds.