Many mechanical systems often show nonlinear behaviour related to particular operating conditions or to the nonlinear characteristic of the elements (springs, dampers, etc.) making up the system. In these cases, common engineering practice is to linearize the equation of motion around a particular operating point, and to design a linear controller. Although this approach is simple, its main disadvantage is that stability properties and validity of the controller are only local. For these reasons, over the last decades, non-linear control techniques have been investigated more and more in order to improve control performance. In particular, in this paper the model-based sliding-mode-control (SMC) technique is considered because of its easy implementation and the considerable robustness of the controller even under significant model uncertainties. This technique is analysed numerically with respect to the pendulum system, to better understand the influence of the control action on the system dynamics, and then experimentally in order to control a highly nonlinear system, consisting of a three-link flexible manipulator. For this system, a nonlinear modal model is developed and a nonlinear observer is designed. Results of experimental tests on the manipulator are also reported.