This book describes physical conditions in the upper atmosphere and magnetosphere of the Earth. The account is introductory, at a level suitable for readers with a basic inderstanding of physics and explains the principles of physical investigation. The principal material covers the neutral and ionized upper atmosphere, the magnetosphere, and structures, dynamics, disturbances and irregularities. This textbook is suitable for advanced undergraduates and new postgraduates who are taking introductory courses in upper atmospheric, ionospheric, or magnetospheric physics.