This paper describes the use of eigenstructure assignment in the direct design of attitude and attitude rate command systems for helicopter flight control. Eigenvalue assignment is used to achieve desired bandwidth based on the handling qualities specifications, and eigenvector assignment is used to achieve decoupling of lateral, longitudinal, heave, and yaw modes and the desired command-response characteristics. Eigenstructure techniques are also used to design a state estimator that gives desired closed-loop frequency response. The stability robustness of the control system is evaluated with respect to an error model that includes rotor, actuator, flexure, and sensor dynamics and computational and sampling delays in the flight control system. The controlled helicopter is shown to exhibit both good frequency and time-response characteristics.