The selectivity of ion channels to different ion species is crucial for important biological processes, as nerve transmission and muscular contraction. Thanks to the availability of experimental atomic structures of several ion channels, including K+-channels, Na+-channels and Ca2+-channels, it is now possible to investigate the microscopic details of conduction and selectivity. As ion conduction is an intrinsically dynamic process, understanding selectivity and conduction requires to reproduce the dynamics of ion channels. Molecular Dynamics simulations are a common strategy to simulate complex biological molecules, as ion channels, at the atomic details. Free energy calculations based on MD simulations revealed how K+-channels might conduct K+ ions at high speed, while still blocking Na+ ions at an astonishing selectivity ratio of 1 Na+ ion every 10 000 K+ ions.