  We explore the constraints on globular cluster formation provided by the
observed conditions in starbursts where globulars are currently forming, and by
the observed properties of young and old globular clusters. We note that the
pressure in the ISM of starbursts and mergers implies that molecular clouds in
these environments have radii similar to those of globular clusters. Such
molecular clouds are therefore viable precursors to globular clusters if the
star formation efficiency in the clouds is high. A high star formation
efficiency may be a consequence of the high density and associated high binding
energy and short dynamical timescale of molecular clouds in such environments.
We also note that the apparent lack of a mass-radius relationship in young and
old globular cluster systems places important constraints on globular cluster
formation models. This is because molecular clouds are observed to follow a
virial scaling relation between mass and radius. We suggest that a variable
star formation efficiency may weaken or eliminate the mass-radius relation of
molecular clouds as they fragment to form globular clusters. We attribute the
absence of young globular clusters in the disks of ordinary galaxies such as
the Milky Way to the relatively low ambient pressures in such systems.
