The classical regulator problem is posed in the context of linear, time-invariant, finite-dimensional systems with deterministic disturbance and reference signals. Control action is generated by a compensator which is required to provide closed loop stability and output regulation in the face of small variations in certain system parameters. It is shown, using the geometric approach, that such a structurally stable synthesis must utilize feedback of the regulated variable, and incorporate in the feedback path a suitably reduplicated model of the dynamic structure of the disturbance and reference signals. The necessity of this control structure constitutes the Internal Model Principle.

It is shown that, in the frequency domain, the purpose of the internal model is to supply closed, loop transmission zeros which cancel the unstable poles of the disturbance and reference signals.

Finally, the Internal Model Principle is extended to weakly nonlinear systems subjected to step disturbances and references.