The formation of jets in a variety of Astrophysical settings is often attributed to the action of magnetic fields, that efficiently tap the rotational energy of the source and accelerate plasma to high bulk velocities. We have examined the problem in both non-relativistic and relativistic flows in the framework of Special Relativity, and we continue the study in the context of General Relativity, trying to understand how the spacetime around a rotating black hole affects the bulk acceleration and collimation of jets.