For precise robot control, endpoint compliance strategies utilize feedback from a force sensor located near the tool/workpiece interface. Such endpoint force control systems have been observed in the laboratory to be limited to unsatisfactory closed-loop performance. This paper discusses the particular dynamic properties of robot systems which can lead to instability and limit performance. A series of lumped-parameter models is developed in an effort to predict the closed-loop dynamics of a force-controlled single axis arm. The models include some effects of robot structural dynamics, sensor compliance, and workpiece dynamics. The qualitative analysis shows that the robot dynamics contribute to force-controlled instability. Recommendations are made for models to be used in control system design.