The universe is the largest system of all. It consists of elementary particles bound together by gravitational, electromagnetic and nuclear forces. Its structural hierar­ chy in space (from atomic nuclei to supergalaxies) and its evolutionary sequence (from the fireball to the diversity of present forms) is governed by the properties of elementary particles and their interactions. This book is an attempt to interpret the structure and evolution of the universe in terms of elementary particles and of their interactions. This book is intended to present a background for students in astronomy and related sciences, such as geophysics, meteorology, plasma physics, chemistry, nuc­ lear physics, space sciences and some others. The universe forms a general framework fo: all the phenomena studied by these sciences. It was possible to squeeze an extensive range of topics from various disciplines into one book of acceptable size only under some severe limitations: (a) no references are given; (b) arguments are shortcut; (c) quantities are often expressed in the order of magnitude; and (d) formulae have been limited to a minimum. Often more hypo­ theses or theories exist for a phenomenon. We have chosen only one. The preference for a theory or hypothesis may be personal and the theory itself may later prove incorrect. But, many theories about a particular phenomenon would cover many pages and might lead to confusing effects.