This is the first book on the subject of quantum-classical analogies. Although quantum and classical physics are founded on very different principles, similarities between quantum and classical phenomena are not uncommon. This fact was considered by physicists as a mere curiosity until, quite recently, it became clear that such analogies between quantum and classical physics can be exploited in theoretical as well as experimental work in emerging research areas such as quantum computing, nano-devices or unconventional light forms (localized light, frozen light). The analogies described in this book include the well-known mathematical similarity between the Schrödinger equation and the Helmholtz equation (and its application in nanodevice design), the similarities between quantum multi-level systems and their optical counterparts, the optical analogue of the uncertainty relation, and the optical implementations of quantum computing algorithms. Acoustic analogies to quantum phenomena are also mentioned. Last but not least, the book describes the mathematical analogies between classical and quantum phase space physics. In sum, these analogies can help to deepen our understanding of quantum mechanical principles, many of which are still hotly debated a century after the foundations of quantum mechanics were laid.