This book describes the basic concepts of various physical phenomena in semiconductors and their modulated structures under high magnetic fields. The topics cover magneto-transport phenomena, cyclotron resonance, far-infrared spectroscopy, magneto-optical spectroscopy, diluted magnetic semiconductors in high magnetic fields, as well as the recent advances in the experimental techniques needed for high field experiments. Starting from the introductory part describing the basic theoretical background, each chapter introduces typical experimental data which were actually obtained in very high magnetic fields mostly in the pulsed field range up to several megagauss (20-100T). The book has both the character of a textbook and a monograph. For researchers and students with an interest in semiconductor physics or in high magnetic fields, it will serve as a useful guide.