Starting from basic principles, the book systematically covers both Heisenberg and Schrödinger realizations of quantum mechanics (in this order). It provides excellent didactic introduction to the essential principles and treats recent concepts such as entanglement and decoherence. The book gives the background needed to understand quantum cryptography, teleportation and computation, and it is especially suitable for introducing the spin. This second edition includes a more friendly presentation to Hilbert spaces, and more practical applications e.g. scanning tunneling microscope (potential barrier).