  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 (like 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 physics do they share? These systems either hydrodynamic or
magnetohydrodynamic (MHD) in nature and are, as such, governed by non-linear
equations. While theoretical models helped us to understand the basic physics
of these objects, numerical simulations have been allowing us to go beyond the
one-dimensional, steady-state approach extracting vital information. In this
lecture, the formation, structure, and evolution of the jets are reviewed with
the help of observational information, MHD and purely hydrodynamical modeling,
and numerical simulations. Possible applications of the models particularly to
YSOs and AGN jets are addressed.
