Effective dynamics of quantum subsystems

A formal framework to describe the effective dynamics of subsystems of isolated quantum systems is set up. It provides for a standard decomposition of this effective dynamics in terms of ingredients of two distinct types, namely (a) a mean-field unitary contribution; and (b) a non-unitary contribution which arises from the dynamical evolution of quantum correlations in the entire system, and which are essentially related to changes of the coherence properties of the subsystem. We show quite generally that this effective dynamics is non-hamiltonian and closely related in form to equations of the transport type.