  Decentralized stochastic control refers to the multi-stage optimization of a
dynamical system by multiple controllers that have access to different
information. Decentralization of information gives rise to new conceptual
challenges that require new solution approaches. In this expository paper, we
use the notion of an \emph{information-state} to explain the two commonly used
solution approaches to decentralized control: the person-by-person approach and
the common-information approach.
