Erratum B. A. Francis and W. M. Wonham: "The Internal Model Principle for Linear Multivariable Regulators," Appl. Math. Opt. 2 (1975), pp. 170–194. The statement of Theorem 2 should be revised to read: "A synthesis is structurally stable at A₃ only if the compensator incorporates an internal model of A₂ which is observable by u and, if (50) holds, controllable by z." This revision removes an apparent circularity at the end of the proof of Proposition 3. The proofs of Theorem 2 and Proposition 3 remain unchanged. W. M. Wonham July 8, 1976Erratum B. A. Francis and W. M. Wonham: "The Internal Model Principle for Linear Multivariable Regulators," Appl. Math. Opt. 2 (1975), pp. 170–194. The statement of Theorem 2 should be revised to read: "A synthesis is structurally stable at A₃ only if the compensator incorporates an internal model of A₂ which is observable by u and, if (50) holds, controllable by z." This revision removes an apparent circularity at the end of the proof of Proposition 3. The proofs of Theorem 2 and Proposition 3 remain unchanged. W. M. Wonham July 8, 1976