Supercapacitors are energy storage systems, complementary to batteries, already used in a number of applications such as regenerative energy braking and start-stop systems. While supercapacitors are already favored for applications requiring large power densities, they still suffer from low energy densities, compared to batteries, which limits their use. Understanding the molecular processes at the origin of the observed performances is essential in order to design optimised systems. In this talk, I will introduce how NMR (Nuclear Magnetic Resonance) can be used as a tool to probe the charge storage mechanisms and ion dynamics in these complex systems. NMR is a very powerful tool for such a task as it can probe separately the various species present in the system and can be used in situ, i.e. while charging and discharging the supercapacitor. I will show what NMR, sometimes combined with appropriate simulation techniques, can tell us about the microscopic events occurring in supercapacitors.