Young stellar objects and Herbig-Haro objects.

Our current understanding of Herbig-Haro objects and their relationship with young stellar outflows is reviewed. The role of multi-waveband observations in the visible, infrared and radio in the study of outflows is discussed with examples drawn from the HH1, HH24 and HH54 outflow complexes. The current theories are briefly considered in the light of spectroscopic and morphological data. It is concluded that although the general mechanism of outflows is understood, the details of the outflowing material's origin and the collimation mechanism remain largely unanswered.