The problem of controlling a manipulator constrained by contact with the environment is described, and a new method for the design of a hybrid position/force controller is developed. A representation of the instantaneous kinematics of a task is derived, and the constraint on a manipulator due to contact uith the environment is analysed. The task is modeled as a set of essential vectors in the directions critical to the performance of the task. The constraint on a manipulator is modelled as a set of admissiblt joint motion vectors, and a set of admissible joint force vectors. A new method for designing a hybrid position/force controller is then developed to control the joints of a manipulator so that the desired task is performed at the end effector using only motions and forces allowed by the constraint. The method can be applied to all type of manipulators constrained due to contact with the environment, including manipulators constrained due to more than one robot grasping a workpiece and manipulators constrained due to the bracing of the arm against, against a work surface. The method is illustrated by a simple three degree of freedom example