It is because the actuator is the final step in the control chain that failure can be so important and hard to compensate for. When the nature or location of the failure is unknown, the offsetting of consequent system uncertainties becomes even more awkward. This monograph centers on counteracting situations in which unknown control inputs become indeterminately unresponsive over an uncertain period of time by adapting the responses of remaining functional actuators. Both "lock-in-place" and varying-value failures are dealt with. The results presented demonstrate: • the existence of nominal plant-model matching controller structures with associated matching conditions for all possible failure patterns; • the choice of a desirable adaptive controller structure; • derivation of novel error models in the presence of failures; • the design of adaptive laws allowing controllers to respond to combinations of uncertainties stemming from activator failures and system parameters.