Evolution has designed free-energy landscapes that confer proteins unique properties. Among these, the ability to withstand mechanical stress or to undergo structural and functional changes upon application of a force. The possibility to apply a precise force to a single protein and monitor its response has also opened a new window on the free-energy landscapes of proteins. In this lecture I will review what I consider the most relevant discoveries brought by the application of force to proteins and show how simulation has been instrumental in explaining and anticipating experimental results and contributed to test and refine theories on protein landscapes and models to predict how such landscapes are affect by a mechanical force.