This book was written for readers interested in learning about the disciplines, methods and results of space research, perhaps because they happened upon the field during the course of their higher education or professional career, or perhaps because they simply feel an urge to know more about the space environment of the Earth. The present monograph is based on lectures cover­ ing the same topic, which have been held regularly over the past years at the University of Bonn. Like the lecture series, the book is directed at a relatively broad group of students and interested laypersons, the only prerequisite being knowledge of fundamental physics and mathematics, as usually acquired from introductory college courses in science or engineering curricula. More specific knowledge is derived in association with each phenomenon considered. These derivations are kept as simple as possible, adhering to the principle that, when conflicts arise, physical insight is preferable to mathematical precision. As a rule, I strived to avoid the trite phrase 'It may be easily shown that . . . ' and tried to present all derivations in readily verifiable steps, even if this may seem somewhat tedious to the more advanced readers. Also serving clarity and insight are the many illustrations, which do indeed often say more than 'a thousand words'. Our knowledge of the Earth's space environment has grown exponentially during the last few decades and an attempt to cover all aspects of the field would extend way beyond the scope of an introductory text.