The jets launched by black holes are widely believed to be a key component of the accretion process. However, despite being discovered decades ago, their physics and energetics are still poorly understood. The past decade has seen a dramatic improvement in the quality of available multi- wavelength data, in terms of both spectral coverage and sensitivity. However, the semi-analytical modelling of jets has advanced slowly, and simple one-zone models are still the preferred method of interpreting data, in particular for AGN jets. These models can roughly constrain the properties of jets but they can not unambiguously couple their emission to the launching regions and internal dynamics, which are usually probed with General Relativistic Magneto-hydrodynamics (GRMHD) simulations. On the other hand, simulations are not easily comparable to observations because they cannot yet self-consistently predict spectra. I will discuss our groups modelling efforts; in particular, we have been developing a more advanced semi-analytical model which accounts for the dynamics of the whole jet, starting from a simplified parametrisation of Relativistic Magneto- hydrodynamics in which the magnetic flux is converted into bulk kinetic energy. I have applied this model to a variety of sources, from super-massive black holes to black hole X-ray binaries. This is the first time that the same physical model has been applied to such a large variety of sources, both in terms of accretion rate and black hole mass. In particular, I will recent results on the M87 radio galaxy, which are complementary to the observations taken by the Event Horizon Telescope.