These lectures present the theory of the nuclear response in the Random Phase Approximation (RP A). In the first lecture, various relations are derived between densities and currents which give rise to the well-known sum rules. Then RPA is derived via the time-dependent Hartree theory. The various formulations of RP A are shown: the configuration space representation, the coordinate space representation, the Landau theory of infinite systems and the RPA for separable interactions constrained by consistency. The remarkable success of RP A in describing the collective density oscillations of closed shell nuclei is illustrated with a few examples. In the final lecture, the 11-c response is discussed with the application of simple theoretical considerations to the empirical data. Finally, we point out several problems which remain in the response theory.