This book fulfills a twofold purpose: to provide a pedagogic panorama of the current microscopic understanding of the basic physics of matter, and to help students acquire a quantitative feeling of the typical orders of magnitude of the main physical quantities (energy, time, temperature, length) involved in the specific conditions relevant for “matter” in its atomic, molecular, and condensed forms. Both tasks are favored by keeping structurally and conceptually well distinct the analysis of the adiabatically separate motions of electrons and of atoms. This distinct treatment is organized in close parallel for molecules and for solids.