The formation of relativistic cosmic jets

I review current ideas on the launching, acceleration, collimation and propagation of relativistic jets and the influence of strong magnetic fields in the process. Recently, several important elements of the entire jet "engine" structure have been shown to play key roles in the production of an astrophysical jet. Depending on the type of system, these include the spin of the central black hole, the thermal and/or magnetic state of the accretion flow, the presence of a re-collimation point in the jet outflow far away from the central object, and the behavior of MHD shocks and kink instabilities in the final jet. While these physical processes probably are at work in all types of relativistic jets (and many even in more benign stellar outflows), I shall concentrate on ones produced by lower luminosity black hole sources, both in active galactic nuclei and in X-ray binaries. I also will discuss the connection between the theoretical concepts and the large body of observational data, now available on these systems.