Active Galactic Nuclei (AGN) are one of the most enigmatic and powerful objects in the universe. They are powered by supermassive black holes and launch highly collimated relativistic jets while accelerating particle to the highest energies. As such, they represent the ideal laboratory to test our current understanding of accretion on to supermassive black holes and jet formation using general relativistic magneto hydrodynamics (GRMHD) simulations. Due to the progress in observational astronomy using the technique of Very Long Baseline Interferometry (VLBI) we can nowadays image and study relativistic jets from their outskirts down to their launching site in the direct vicinity of supermassive black holes. Making use of the VLBI technique at 1.3 mm wavelength the recent observations of the Event Horizon Telescope (EHT) can resolve and image the accretion flow around the supermassive black hole in the giant elliptical galaxy Messier 87 (M 87) and in the galactic center Sagittarius A* (SgrA \(^\star \) ). Using the Global Millimetre VLBI Array (GMVA) at 3.5 mm the propagation of relativistic jets can be observed and monitored with great details. There is vast and growing literature on methods and tools for comparing GRMHD simulations to various observations and sources. In order to provide an pedagogical introduction to the field we will focus in this Chapter on how to compare GRMHD simulations with recent M 87 observations of the EHT and GMVA.