The motivation of my work is to understand the role played by the magnetic field in the dynamics and emission of relativistic jets. In order to achieve this objective I have been carrying it out in two parallel ways. In the first of them I have performed numerical magnetohydrodynamic and emission (eRMHD) simulations of relativistic jets. The simulations have been performed in collaboration with the Relativistic Astrophysics Group in the University of Valencia, using a numerical code that solves the RMHD equations in conservative form and cylindrical coordinates with axial symmetry (see Leismann et al., 2005, for more details). I have focused the study on the role played by the magnetic field in the dynamics of the jet, analyzing the balance of the main driving forces which determine the jet evolution. By using these relativistic magnetohydrodynamic (RMHD) simulations as input I have computed the non-thermal (synchrotron) emission which allows to obtain synthetic radio maps that can be directly compared with actual observations (Roca-Sogorb et al., 2008a,b, 2009). The synergy between simulations and observations is proven to be a powerful tool in the under- standing of the physical processes taking place in jets. For this, my second line of work has been the comparison of the eRMHD results with actual sources. I have started the study with a very well known source: the radiogalaxy 3C 120. I have carried out new observations, taken in November 2007 making use of all available observing frequencies (from 1 to 86 GHz) with the VLBA and VLA. These observations, taken also in polarimetric mode, allow to study the jet in 3C120 from pc to Kpc scales with great detail. The comparison of these observations with those from 1999 to 2001 (Gomez et al., o 2008) provides information about the source of Faraday rotation in the jet of the radio galaxy 3C 120. The results indicate that the emitting jet and the source of Faraday rotation are not closely connected physically and have different configurations for the magnetic field and/or kinematical properties, favoring a model in which a significant fraction of the RM originates in foreground clouds (Gomez et al., o 2011). The higher frequency 2007 observations reveal a new component located 80 mas from the core (which corresponds to a deprojected distance of 140 pc), with a brightness temperature about 600 times higher than expected at such distances. I have analyzed the different processes that could be responsible for the enhanced brightness temperature observed, its sudden appearance, and apparent 1 stationary (Roca-Sogorb et al., 2010).