As specialized robotic manipulators have been designed to lift and accurately move small loads, the need for machines with lightweight, low inertia links has been noted. Such designs imply that some structural rigidity must be sacrificed, shifting the mechanical resonance spectrum lower in frequency toward the frequencies of interest to the system controller. These lower resonant frequencies with equal or lower damping cause larger absolute end-effector overahoot, in the absence of compensation schemea. One method employed to regain system performance, the observer- controller, involves implementation of a control law utilizing feedback from estimates of all the states of a model of the link including those states not directly measurable. A particular example of the observer-controller based on a reduced order model of a single link flexible arm is developed in this paper.