  Highly collimated supersonic jets and less collimated outflows are observed
to emerge from a wide variety of astrophysical objects. They are seen in young
stellar objects (YSOs), proto-planetary nebulae, compact objects (such as
galactic black holes or microquasars, and X-ray binary stars), and in the
nuclei of active galaxies (AGNs). Despite their different physical scales (in
size, velocity, and amount of energy transported), they have strong
morphological similarities. What is the universal mechanism that can explain
their origin? In this lecture, I briefly review the role that magnetic fields
seem to play on the formation, structure, and propagation of these jets.
