  Quantum decoherence plays a pivotal role in the dynamical description of the
quantum-to-classical transition and is the main impediment to the realization
of devices for quantum information processing. This paper gives an overview of
the theory and experimental observation of the decoherence mechanism. We
introduce the essential concepts and the mathematical formalism of decoherence,
focusing on the picture of the decoherence process as a continuous monitoring
of a quantum system by its environment. We review several classes of
decoherence models and discuss the description of the decoherence dynamics in
terms of master equations. We survey methods for avoiding and mitigating
decoherence and give an overview of several experiments that have studied
decoherence processes. We also comment on the role decoherence may play in
interpretations of quantum mechanics and in addressing foundational questions.
