Historically, the theoretical and observational aspects of relativistic jets have been seen as somewhat related, but overall distinct. Indeed, each of the different aspects of jet physics has been addressed separately from the others and as isolated problems rather than elements of a single, bigger picture. For example, jet-launching has been modelled separately from jet propagation and stability, just like the latter has been treated after assuming that some “magic” central engine has provided the acceleration and energy needed for propagation. JETSET will depart from this perspective in favour of an approach – never tried before – in which all of these aspects are tackled simultaneously for a genuinely comprehensive understanding of relativistic jets. JETSET will develop and make use of a single computational infrastructure providing a unified virtual environment to simulate first the launching and acceleration processes where strong-gravity effects near the black hole dominate; then the more distant propagation regions where special relativity effects dominate and a number of instabilities threaten the dynamics; and finally the breakout regions where the jet interacts with the ambient medium producing the observed phenomenology. These different stages of a relativistic jet’s life are described in the figure below, and each of these stages is the focus of one work package of JETSET. In JETSET, theorists work closely with observers to construct novel synergies between the two major approaches used to study relativistic jets so far.