  We develop a general framework for the steady-state response of dissipative
quantum systems. We concretely derive three different, but equivalent, forms of
the quantum response function. We discuss for each of them the role of the
noncommutativity of quantum operators and establish links to the Kubo response
theory for closed quantum systems. We show in particular that the equilibrium
response vanishes for perturbations that commute with the unperturbed
Hamiltonian while the steady-state response does not, highlighting the profound
difference between the two.
