  The interstellar molecular clouds are the site of star formation and also the
target for the cosmic ray protons to produce \gamma-rays via the hadronic
process. The interstellar atomic gas is enveloping the molecular clouds and may
also be dense enough to affect the \gamma-ray production. In this Chapter, some
of the basic properties of the interstellar gas both in molecular and atomic
forms will be reviewed. Then, it is presented that two young TeV \gamma-ray
SNRs, RX J1713.7-3946 and RX J0852.0-4622, show good spatial correspondence
between the \gamma-rays and the interstellar protons. The good spatial
correspondence provides a support for the hadronic origin of the \gamma-rays in
these SNRs. It is emphasized that both molecular and atomic hydrogen plays a
role as targets for cosmic ray (CR) protons. The clumpy distribution of the
target interstellar medium (ISM) protons is crucial in the interaction of the
supernova shocks with the ISM, whereas models with uniform ISM distribution are
not viable. Finally, it is suggested that the dense atomic gas without
molecules may occupy the dominant part of the dark gas in the local ISM.
