  We review some aspects of the bipolar molecular outflow phenomenon. In
particular, we compare the morphological properties, energetics and velocity
structures of outflows from high and low-mass protostars and investigate to
what extent a common source model can explain outflows from sources of very
different luminosities. Many flow properties, in particular the CO spatial and
velocity structure, are broadly similar across the entire luminosity range,
although the evidence for jet-entrainment is still less clear cut in massive
flows than in low-mass systems. We use the correlation of flow momentum
deposition rate with source luminosity to estimate the ratio f of mass ejection
to mass accretion rate. From this analysis, it appears that a common driving
mechanism could operate across the entire luminosity range. However, we stress
that for the high-mass YSOs, the detailed physics of this mechanism and how the
ejected wind/jet entrains ambient material remain to be addressed. We also
briefly consider the alternative possibility that high-mass outflows can be
explained by the recently proposed circulation models, and discuss several
shortcomings of those models. Finally, we survey the current evidence on the
nature of the shocks driven by YSOs during their pre-main-sequence evolution.
