The problem of controlling the output of a system so as to achieve asymptotic tracking of prescribed trajectories and/or asymptotic rejection of undesired disturbances is a central problem in control theory. In the standard formulation of output regulation, one assumes knowledge of an exosystem which generates signals to be tracked and disturbances to be attenuated. The case when the exogenous system is a harmonic oscillator is, of course, classical. In this case, to solve the output regulation problem is to track, or attenuate, a sinusoid of known frequency but with unknown amplitude and phase, using only error measurement feedback. This is in sharp contrast with alternative approaches, such as exact output tracking, where in lieu of the assumption that a signal is within a class of signals generated by an exogenous system, one instead assumes complete knowledge of the past, present, and future time history of the trajectory to be tracked. The book gives a unified treatment of output regulation theory for linear and nonlinear systems and addresses a number of issues which were left open in the earlier works on the subject. In particular, it describes an approach to structurally stable regulation which unifies and extends a number of prior existing results. It also addresses the issue of robust regulation, i.e., the issue of achieving regulation in the presence of parameter uncertainties ranging within a prescribed set.