Quantum coherence is a phenomenon that plays a crucial role in various forms of matter. The thriving field of quantum information, as well as unconventional approaches to use mesoscopic systems in future optoelectronic devices, provide the exciting background for this set of lectures. The lectures originate from the well-known Schladming Winter Schools and are carefully edited so as to address a broad readership ranging from the beginning graduate student up to the senior scientist wanting to keep up with or to enter newly emerging fields of research.