The study of open quantum systems is concerned with understanding and predicting the dynamics of quantum systems that are coupled significantly to their surroundings, leading to effects such as dissipation and decoherence.

All quantum systems are coupled to their surroundings to some extent. In some cases, this coupling is so weak that the system can be treated as approximately isolated, so that the usual laws of unitary quantum evolution apply. However, in many cases, the influence of the environment cannot be ignored and the system no longer evolves unitarily, in which case the quantum system is called open. The study of open quantum systems is concerned with understanding and predicting the dynamics of such open systems, including the most important effects arising due to their unavoidable interaction with the environment, such as decoherence and energy dissipation.