This book gathers the lecture notes of courses given at the 2011 summer school in theoretical physics in Les Houches‎,‎ France‎,‎ Session XCVI‎.‎ What is a quantum machine‎?‎ Can we say that lasers and transistors are quantum machines‎?‎ After all‎,‎ physicists advertise these devices as the two main spin‎-offs of the understanding of quantum mechanical phenomena‎.‎ However‎,‎ while quantum mechanics must be used to predict the wavelength of a laser and the operation voltage of a transistor‎,‎ it does not intervene at the level of the signals processed by these systems‎.‎ Signals involve macroscopic collective variables like voltages and currents in a circuit or the amplitude of the oscillating electric field in an electromagnetic cavity resonator‎.‎ In a true quantum machine‎,‎ the signal collective variables‎,‎ which both inform the outside on the state of the machine and receive controlling instructions‎,‎ must themselves be treated as quantum operators‎,‎ just as the position of the electron in a hydrogen atom‎.‎ Quantum superconducting circuits‎,‎ quantum dots‎,‎ and quantum nanomechanical resonators satisfy the definition of quantum machines‎.‎ These mesoscopic systems exhibit a few collective dynamical variables‎,‎ whose fluctuations are well in the quantum regime and whose measurement is essentially limited in precision by the Heisenberg uncertainty principle‎.‎ Other engineered quantum systems based on natural‎,‎ rather than artificial degrees of freedom can also qualify as quantum machines‎:‎ trapped ions‎,‎ single Rydberg atoms in superconducting cavities‎,‎ and lattices of ultracold atoms‎.‎ This book provides the basic knowledge needed to understand and investigate the physics of these novel systems‎.‎