Spectroscopic studies of Van der Waals molecules have recently provided detailed and precise information on the forces between molecules and in particular rare-gas atoms. This information can be used to calculate properties of condensed phases, thus providing a stringent test of theoretical and computational methods. Condensed phases may themselves be regarded as very large Van der Waals molecules, and smaller Van der Waals molecules (clusters) play an important role in the nucleation and growth of condensed phases. This paper reviews these questions with particular reference to rare gases.