  We collect recent results on deriving useful response relations also for
nonequilibrium systems. The approach is based on dynamical ensembles,
determined by an action on trajectory space. (Anti)Symmetry under time-reversal
separates two complementary contributions in the response, one entropic the
other frenetic. Under time-reversal invariance of the unperturbed reference
process, only the entropic term is present in the response, giving the standard
fluctuation-dissipation relations in equilibrium. For nonequilibrium reference
ensembles, the frenetic term contributes essentially and is responsible for new
phenomena. We discuss modifications in the Sutherland-Einstein relation, the
occurence of negative differential mobilities and the saturation of response.
We also indicate how the Einstein relation between noise and friction gets
violated for probes coupled to a nonequilibrium environment. We end with some
discussion on the situation for quantum phenomena, but the bulk of the text
concerns classical mesoscopic (open) systems. The choice of many simple
examples is trying to make the notes pedagogical, to introduce an important
area of research in nonequilibrium statistical mechanics.
