The book is essentially a restatement of existing knowledge, contributing with new perspectives and points of view in the presentation and discussion of the different contents. The aim of the book is not replacing any of the various books dealing with Physical Chemistry existing nowadays, but to complement the explanation of subjects making use of an interdisciplinary approach, stressing their application to other areas of science. We are sure this will be the near future role of Physical Chemistry. So our book will encompass the description of molecular interactions and effects of size in the temporal and spatial scales of the processes described. This book is to a large extent a translation of “Moléculas y Materia”, EUBEBA (Buenos Aires, Argentina, 2nd Edition and re-impression, 2010). The English version will have a few additions compared to the Spanish edition, especially to the last chapter “Future Development”.