It is shown how the results of compressibility measurements in gases and liquids lead to the hypothesis of molecular interaction, and how details about this interaction may be obtained from the experimental data for the equation of state. The reasons for the molecular interaction are found in the laws of electrostatics and quantum mechanics.The problems the molecular physicist is interested in are threefold. He wants to understand molecular interaction from first principles, he tries to get information about this interaction from a variety of experiments, and he seeks to understand a large group of experimental observations once the interaction of the molecules is known. For the solution of the second and third problem it is usually necessary to link the macroscopic observables to the microscopic molecular properties; although this has been possible in some instances, the general problem cannot be solved.