As a preliminary step toward linear response theory, the Kubo relation for the Brownian particle is described. The generalization of the fluctuation formalism to generalized thermodynamic observables is also illustrated, providing an explicit approach to linear response to external static, as well as time-dependent perturbation fields. Generalized fluctuation–dissipation relations are also introduced by this formalism. The Onsager regression relation is discussed as a basis for a general theory of transport processes, including coupled-transport phenomena.