This paper highlights the importance of considering the piezoelectric constraining layer voltage (or electric field) limits when evaluating the effectiveness of an active constrained layer damping treatment in attenuating resonant vibration. It is seen that, when position feedback is used, intermediate viscoelastic layer stiffness values are always optimal, and maximum allowable control gains and possible vibration attenuation progressively decrease with increasing excitation force levels. On the other hand, with velocity feedback, the optimal viscoelastic layer stiffness is dependent on the excitation level. For low excitation force amplitudes, stiff viscoelastic layers are most effective, with large velocity feedback gains producing substantial vibration attenuation without exceeding piezoelectric layer voltage limits. However, for higher excitation force levels, stiff viscoelastic layers result in excess voltages even at very small velocity feedback gains, and are unable to provide any vibration attenuation. In such a case, intermediate viscoelastic layer stiffness values are preferable, and maximum velocity feedback gains and possible vibration attenuation progressively decrease with increasing excitation level, as in the case of position feedback. For both position and velocity feedback, when excitation forces are beyond a certain level the allowable control gains are so limited that no additional damping is obtained beyond that already available through the passive treatment.