  There are about 70 Galactic supernova remnants (SNRs) that are now confirmed
or suggested to be in physical contact with molecular clouds (MCs) with six
kinds of evidence of multiwavelength observations. Recent detailed CO-line
spectroscopic mappings of a series of SNRs reveal them to be in cavities of
molecular gas, implying the roles the progenitors may have played. We predict a
linear correlation between the wind bubble sizes of main-sequence OB stars in a
molecular environment and the stellar masses and discuss its implication for
supernova progenitors. The molecular environments of SNRs can serve as a good
probe for the gamma-rays arising from the hadronic interaction of the
accelerated protons, and this paper also discusses the gamma-ray emission from
MCs illuminated by diffusive protons that escape from SNR shocks.
