Abstract: This paper presents a versatile approach to the synthesis of accurate state observers for flexible link mechanisms and manipulators. The design of a state observer should be always based on a model providing an adequate description of the system dynamics. However, when flexible link mechanisms are considered, the synthesis of a state observer becomes very challenging, since only nonlinear models may be adopted to reproduce the system dynamic response with adequate accuracy. In this work the possibility of employing a Kalman estimator together with a suitable piecewise-linear model is investigated. Numerical results prove the effectiveness of the proposed approach when it is applied to the synthesis of a state observer for a four-bar linkage with all the links flexible.