Robot manipulators in which the joints exhibit a certain amount of elasticity are discussed. The model of such robots is highly nonlinear, and available techniques to build nonlinear observers generally do not apply. A procedure to construct approximate nonlinear observers is proposed that uses a recent approach to deriving observers based on geometric nonlinear control theory together with an approximation technique. The conditions under which an exact observer exists are derived. Some examples are given for which simulation tests assure a good performance even when the observer is connected to a control system. >