  We present the results of numerical hydrodynamic models for the collimation
of outflows from young stellar objects. We show that the presence of a toroidal
environment can lead to efficient formation of jets and bipolar outflows from
initially uncollimated central winds. The interaction between the wind and the
environment leads to two types of collimation, one which is dominated by
radiative cooling effects, and one which works when cooling is less efficient.
We describe the two types of jets as they appear in the simulations and we
suggest a description for the long term evolution of these structures in more
realistic time-dependent wind sources.
