Tunneling is a genuine quantum effect, a direct consequence of the matter wave structure of quantum mechanics. Recent progress in engineering and manufacturing aggregates on the nanoscopic and mesoscopic scale have led to fascinating developments in directly influencing and controlling quantum properties in general, and tunneling in particular. In parallel, an exciting exchange of experimental techniques and theoretical concepts from fields such as atomic, molecular, and condensed matter physics has emerged. The aim of this book is to provide a survey of one of the most powerful theoretical tools to describe tunneling, namely, the semiclassical approximation, and to show that tunneling phenomena are central issues in this fast rising interdisciplinary field.