In Leiden, we study a set of fascinating quantum systems where interactions are strong, ranging from single molecules via Van der Waals materials to correlated-electron oxides. Their properties depend both on the states (bands) the electrons occupy and on the interactions between the electrons themselves. Remarkably, these properties may vary locally, on the nanometer scale. Hence, we investigate these systems by using highly advanced techniques that combine microscopy with spectroscopy (energy-dependent measurements), e.g. charge transport spectroscopy, molecular fluorescence spectroscopy, STM spectroscopy and LEEM spectroscopy. Several of these techniques have been improved or even designed in our groups, in order to continuously push the limits of our experimental capabilities, e.g. by increasing energy resolution or decreasing the operation temperatures.