In general, structures represent distributed-parameter systems described by partial differential equations. Strictly speaking, vibration control in distributed structures requires distributed sensors and actuators. Practical considerations demand that control be carried out by means of discrete components. Quite often they also demand that the structure be discretized in space. A discretization procedure having particular appeal is the finite element method, which has the advantage that the nodal (in the finite element sense) coordinates represent actual displacements of the structure. One of the most significant problems in feedback control is the generation of control gains. Techniques for computing gains in common use are the pole allocation method and optimal control. These methods work best for low-order systems. This tends to create a problem in structures, which generally demand high-order mathematical models. Feedback control requires determination of the system state, which can be done through measurement of the nodal displacements and velocities. If it is not feasible to measure all the components of the state, then it is possible to construct a dynamical system, related to the actual system and known as a Luenberger observer, to estimate the full state. One method of controlling the vibration of a structure is modal control, which implies controlling the modes of vibration of the structure. In the independent modal-space control method, design of the control can be carried out for each mode independently. This requires estimation of the modal states. A method not requiring estimation of the modal states is direct feedback control, in which the sensor signals are amplified directly to generate the feedback forces. Under given circumstances, direct feedback can be rendered nearly optimal.