Summary: Systematic approaches to select the optimal candidate sets for actuator and exciter placement are developed. The performance of control and system identification for flexible structures depends strongly on the locations of actuators and exciters. The proposed methods of optimal actuator and exciter placement rely on quantitative measures of modal degree of controllability and modal degree of excitability, respectively. Whereas modal degree of controllability is related to the minimum control input energy needed to regulate the system from initial modal disturbances, modal degree of excitability is related to the steady-state energy stored in each mode when white noise is applied to excite the system. The modal degrees of controllability and excitability offer the control system designer tools that allow the ranking of the effectiveness of a specific distribution of actuators and exciters and, hence, the choice of their locations on a rational basis. The effectiveness of the proposed measures is demonstrated by both computer simulation and an experiment.