  The role of large-scale stellar feedback in the formation of molecular clouds
has been investigated observationally by examining the relationship between HI
and 12CO(J=1-0) in supershells. Detailed parsec-resolution case studies of two
Milky Way supershells demonstrate an enhanced level of molecularisation over
both objects, and hence provide the first quantitative observational evidence
of increased molecular cloud production in volumes of space affected by
supershell activity. Recent results on supergiant shells in the LMC suggest
that while they do indeed help to organise the ISM into over-dense structures,
their global contribution to molecular cloud formation is of the order of only
~10%.
