This review seeks to describe, from first principles, the nature of the interaction forces in atomic and ionic liquids. The atoms and molecules made up of dipoles and multipoles interact with van der Waals forces, while the ionic systems are viewed as pseudoions interacting through effective forces depending on the electronic structure and the physical ionic arrangement. The interplay between these two aspects of materials is quite complex and forms the main subject of this review. As it will be shown, the two-component system of interacting electrons and ions can be reduced, in second order perturbation theory, to an effective one-component system made up of pseudoions acting under the influence of two-body, central, screened potentials. These potentials result from a weak interaction between the electrons and the ions, deduced from the pseudopotential theory. Once the interatomic forces are known, the atomic structure and the electronic transport properties can be determined by methods of classical mechanics and quantum mechanics. Besides, a large volume-dependent term in the free energy, independent of the ionic positions, which distinguishes the conducting liquids from the simple isolator liquids like argon, is indispensable for explaining the thermodynamical properties.