Nonadiabatic transition is a highly multi-disciplinary concept and phenomenon, constituting a fundamental mechanism of state and phase changes in various dynamical processes of physics, chemistry and biology. This book is intended to be readable to a broad audience so that they can deepen their understanding of the basic concepts of both time-independent and time-dependent nonadiabatic transitions. Quantum mechanically intriguing phenomena such as complete reflection and nonadiabatic tunneling are emphasized. The Zhu-Nakamura theory that can deal with non-negligible classically forbidden transitions is explained. Furthermore, by controlling nonadiabatic transitions induced by an external field such as laser, designing chemical reaction dynamics as we desire is shown to be theoretically possible.